minor #8115 Added an introductory article for Symfony Flex (javiereguiluz)

This PR was squashed before being merged into the 3.3 branch (closes #8115).

Discussion
----------

Added an introductory article for Symfony Flex

We need some official doc about Flex as soon as possible. Later we'll add more articles about it as needed. Thanks!

Commits
-------

dba7c3d Added an introductory article for Symfony Flex

Author: xabbuh

setup/flex.rst

@@ -0,0 +1,173 @@
+.. index:: Flex
+
+Using Symfony Flex to Manage Symfony Applications
+=================================================
+
+`Symfony Flex`_ is the new way to install and manage Symfony applications. Flex
+is not a new Symfony version, but a tool that replaces and improves the
+`Symfony Installer`_.
+
+Symfony Flex **automates the most common tasks of Symfony applications**, such
+as installing and removing bundles and other dependencies. Symfony Flex works
+for Symfony 3.3 and newer versions. Starting from Symfony 4.0, Flex will be used
+by default, but it will still be optional to use.
+
+How Does Flex Work
+------------------
+
+Internally, Symfony Flex is a Composer plugin that modifies the behavior of the
+``require`` and ``update`` commands. When installing or updating dependencies
+in a Flex-enabled application, Symfony can perform tasks before and after the
+execution of Composer tasks.
+
+Consider the following example:
+
+.. code-block:: terminal
+
+    $ cd my-project/
+    $ composer require mailer
+
+If you execute that command in a Symfony application which doesn't use Flex,
+you'll see a Composer error explaining that ``mailer`` is not a valid package
+name. However, if the application has Symfony Flex installed, that command ends
+up installing and enabling the SwiftmailerBundle, which is the best way to
+integrate Swiftmailer, the official mailer for Symfony applications.
+
+When Symfony Flex is installed in the application and you execute ``composer require``,
+the application makes a request to Symfony Flex servers before trying to install
+the package with Composer:
+
+* If there's no information about that package, Flex server returns nothing and
+  the package installation follows the usual procedure based on Composer;
+* If there's special information about that package, Flex returns it in a file
+  called "recipe" and the application uses it to decide which package to install
+  and which automated tasks to run after the installation.
+
+In the above example, Symfony Flex asks about the ``mailer`` package and the
+Symfony Flex server detects that ``mailer`` is in fact an alias for SwiftmailerBundle
+and returns the "recipe" for it.
+
+Symfony Flex Recipes
+~~~~~~~~~~~~~~~~~~~~
+
+Recipes are defined in a ``manifest.json`` file and can contain any number of
+other files and directories. For example, this is the ``manifest.json`` for SwiftmailerBundle:
+
+.. code-block:: javascript
+
+    {
+        "bundles": {
+            "Symfony\\Bundle\\SwiftmailerBundle\\SwiftmailerBundle": ["all"]
+        },
+        "copy-from-recipe": {
+            "config/": "%CONFIG_DIR%/"
+        },
+        "env": {
+            "MAILER_URL": "smtp://localhost:25?encryption=&auth_mode="
+        },
+        "aliases": ["mailer", "mail"]
+    }
+
+The ``alias`` option allows Flex to install packages using short and easy to
+remember names (``composer require mailer`` vs ``composer require symfony/swiftmailer-bundle``).
+The ``bundles`` option tells Flex in which environments should this bundle be
+enabled automatically (``all`` in this case). The ``env`` option makes Flex to
+add new environment variables to the application. Finally, the ``copy-from-recipe``
+option allows the recipe to copy files and directories into your application.
+
+The instructions defined in this ``manifest.json`` file are also used by Symfony
+Flex when uninstalling dependencies (e.g. ``composer remove mailer``) to undo
+all changes. This means that Flex can remove the SwiftmailerBundle from the
+application, delete the ``MAILER_URL`` environment variable and any other file
+and directory created by this recipe.
+
+Symfony Flex recipes are contributed by the community and they are stored in
+two public repositories:
+
+* `Main recipe repository`_, is a curated list of recipes for high quality and
+  maintained packages. Symfony Flex only looks in this repository by default.
+* `Contrib recipe repository`_, contains all the recipes created by the community.
+  All of them are guaranteed to work, but their associated packages could be
+  unmaintained. Symfony Flex ignores these recipes by default, but you can execute
+  this command to start using them in your project:
+
+  .. code-block:: terminal
+
+        $ cd your-project/
+        $ composer config extra.symfony.allow-contrib true
+
+Read the `Symfony Recipes documentation`_ to learn everything about how to
+create recipes for your own packages.
+
+Using Symfony Flex in New Applications
+--------------------------------------
+
+Symfony has published a new "skeleton" project, which is a minimal Symfony
+project recommended to create new applications. This "skeleton" already includes
+Symfony Flex as a dependency, so you can create a new Flex-enabled Symfony
+application executing the following command:
+
+.. code-block:: terminal
+
+    $ composer create-project symfony/skeleton my-project
+
+.. note::
+
+    The use of the Symfony Installer to create new applications is no longer
+    recommended since Symfony 3.3. Use Composer ``create-project`` command instead.
+
+Upgrading Existing Applications to Flex
+---------------------------------------
+
+Using Symfony Flex is optional, even in Symfony 4, where Flex will be used by
+default. However, Flex is so convenient and improves your productivity so much
+that it's strongly recommended to upgrade your existing applications to it.
+
+The only caveat is that Symfony Flex requires that applications use the
+following directory structure, which is the same used by default in Symfony 4:
+
+.. code-block:: text
+
+    your-project/
+    ├── Makefile
+    ├── config/
+    │   ├── bundles.php
+    │   ├── container.yaml
+    │   ├── packages/
+    │   └── routing.yaml
+    ├── public/
+    │   └── index.php
+    ├── src/
+    │   ├── ...
+    │   └── Kernel.php
+    ├── templates/
+    └── vendor/
+
+This means that installing the ``symfony/flex`` dependency in your application
+is not enough. You must also upgrade the directory structure to the one showed
+above. Sadly, there's no automatic tool to make this upgrade, so you must follow
+these manual steps:
+
+1. Create a new empty Symfony application (``composer create-project symfony/skeleton my-project-flex``)
+2. Copy the ``require`` and ``require-dev`` dependencies defined in your original
+   project's ``composer.json`` file to the ``composer.json`` file of the new project.
+3. Install the dependencies in the new project executing ``composer install``. This
+   will make Symfony Flex generate all the configuration files in ``config/packages/``
+4. Review the generated ``config/packages/*.yaml`` files and make any needed
+   changes according to the configuration defined in the ``app/config/config_*.yml``
+   file of your original project. Beware that this is the most time-consuming
+   and error-prone step of the upgrade process.
+5. Move the original parameters defined in ``app/config/parameters.*.yml`` to the
+   new ``config/container.yaml`` and ``.env`` files depending on your needs.
+6. Move the original source code from ``src/{App,...}Bundle/`` to ``src/`` and
+   update the namespaces of every PHP file (advanced IDEs can do this automatically).
+7. Move the original templates from ``app/Resources/views/`` to ``templates/``
+8. Make any other change needed by your application. For example, if your original
+   ``web/app_*.php`` front controllers were customized, add those changes to the
+   new ``public/index.php`` controller.
+
+.. _`Symfony Flex`: https://github.com/symfony/flex
+.. _`Symfony Installer`: https://github.com/symfony/symfony-installer
+.. _`Main recipe repository`: https://github.com/symfony/recipes
+.. _`Contrib recipe repository`: https://github.com/symfony/recipes-contrib
+.. _`Symfony Recipes documentation`: https://github.com/symfony/recipes/blob/master/README.rst