feature symfony#4236 Javiereguiluz bundle install instructions (WouterJ)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#4236).

Discussion
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Javiereguiluz bundle install instructions

Replaces symfony#4163

Commits
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7097070 Added the fixes suggested by Ryan
35dddba Reworded a bit the installation instructions
7a6644b Removed the sidebar and made some minor tweaks
41ce2a5 Minor fixes and tweaks
4b29de2 More tweaks and fixes
322ec60 Removed the first person perspective and other minor fixes
c39f7d3 Fixed some code formatting
ed96447 More improvements based on reviewers' comments
21ce2c0 Fixed minor typo
11db860 Applied all the fixes and suggestions made by reviewers
b297d29 First draft of the bundle installation instructions

Author: weaverryan

cookbook/bundles/best_practices.rst

@@ -194,6 +194,59 @@ Extensive documentation should also be provided in the
 the ``Resources/doc/`` directory; the ``Resources/doc/index.rst`` file is
 the only mandatory file and must be the entry point for the documentation.
 
+Installation Instructions
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to ease the installation of third-party bundles, consider using the
+following standardized instructions in your ``README.md`` file.
+
+.. code-block:: text
+
+    Installation
+    ============
+
+    Step 1: Download the Bundle
+    ---------------------------
+
+    Open a command console, enter your project directory and execute the
+    following command to download the latest stable version of this bundle:
+
+    ```bash
+    $ composer require <package-name> "~1"
+    ```
+
+    This command requires you to have Composer installed globally, as explained
+    in the [installation chapter](https://getcomposer.org/doc/00-intro.md)
+    of the Composer documentation.
+
+    Step 2: Enable the Bundle
+    -------------------------
+
+    Then, enable the bundle by adding the following line in the `app/AppKernel.php`
+    file of your project:
+
+    ```php
+    <?php
+    // app/AppKernel.php
+
+    // ...
+
+    public function registerBundles()
+    {
+        $bundles = array(
+            // ...
+            return new <vendor>\<bundle-name>\<bundle-long-name>(),
+        );
+    }
+    ```
+
+This template assumes that your bundle is in its ``1.x`` version. If not, change
+the ``"~1"`` installation version accordingly (``"~2"``, ``"~3"``, etc.)
+
+Optionally, you can add more installation steps (*Step 3*, *Step 4*, etc.) to
+explain other required installation tasks, such as registering routes or
+dumping assets.
+
 Routing
 -------