Merge branch '3.4' into 4.2

* 3.4:
  added "special case"

Author: javiereguiluz

contributing/code/pull_requests.rst

@@ -131,6 +131,12 @@ work:
 
 * ``master``, if you are adding a new feature.
 
+  The only exception is when a new :doc:`major Symfony version </contributing/community/releases>`
+  (4.0, 5.0, etc.) comes out every two years. Because of the
+  :ref:`special development process <major-version-development>` of those versions,
+  you need to use the previous minor version for the features (e.g. use ``3.4``
+  instead of ``4.0``, use ``4.4`` instead of ``5.0``, etc.)
+
 .. note::
 
     All bug fixes merged into maintenance branches are also merged into more
@@ -423,3 +429,4 @@ before merging.
 .. _`Symfony Slack`: https://symfony.com/slack-invite
 .. _`Travis-CI`: https://travis-ci.org/symfony/symfony
 .. _`draft status`: https://help.github.com/en/articles/about-pull-requests#draft-pull-requests
+.. _`Symfony Roadmap`: https://symfony.com/roadmap

contributing/community/releases.rst

@@ -84,6 +84,23 @@ adds a new preferred one along side. Read the
 :ref:`conventions <contributing-code-conventions-deprecations>` document to
 learn more about how deprecations are handled in Symfony.
 
+.. _major-version-development:
+
+This deprecation policy also requires a custom development process for major
+versions (4.0, 5.0, 6.0, etc.) In those cases, Symfony develops at the same time
+two versions: the new major one (e.g. 4.0) and the latest version of the
+previous branch (e.g. 3.4).
+
+Both versions have the same new features, but they differ in the deprecated
+features. The oldest version (3.4 in this example) contains all the deprecated
+features whereas the new version (4.0 in this example) removes all of them.
+
+This allows you to upgrade your projects to the latest minor version (e.g. 3.4),
+see all the deprecation messages and fix them. Once you have fixed all those
+deprecations, you can upgrade to the new major version (e.g. 4.0) without
+effort, because it contains the same features (the only difference are the
+deprecated features, which your project no longer uses).
+
 Rationale
 ---------