minor #5578 Add supported branches in platform.sh section (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Add supported branches in platform.sh section

Replaces #5532 with a solution that is less likely to get outdated.

| Q | A
| --- | ---
| Doc fix? | yes
| New docs? | yes
| Applies to | all
| Fixed tickets | -

Commits
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f33868b Improve Platform.sh mention in contributing docs

contributing/documentation/overview.rst

@@ -269,6 +269,11 @@ page on GitHub and click on ``Details``.
     ``.platform.app.yaml``, ``.platform/services.yaml`` and 
     ``.platform/routes.yaml`` allow `Platform.sh`_ to build Pull Requests.
 
+.. note::
+
+    Only Pull Requests to maintained branches are automatically built by
+    Platform.sh. Check the `roadmap`_ for maintained branches.
+
 Minor Changes (e.g. Typos)
 --------------------------
 
@@ -345,4 +350,5 @@ definitely don't want you to waste your time!
 .. _SensioLabsConnect: https://connect.sensiolabs.com/
 .. _`Symfony Documentation Badge`: https://connect.sensiolabs.com/badge/36/symfony-documentation-contributor
 .. _`sync your fork`: https://help.github.com/articles/syncing-a-fork
-.. _`Platform.sh`: https://platform.sh
+.. _`Platform.sh`: https://platform.sh
+.. _`roadmap`: https://symfony.com/roadmap