bug #4513 [Contributing] update contribution guide for 2.7/3.0 (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Contributing] update contribution guide for 2.7/3.0

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4508

Commits
-------

87582a4 update contribution guide for 2.7/3.0

contributing/code/patches.rst

@@ -105,9 +105,13 @@ Choose the right Branch
 ~~~~~~~~~~~~~~~~~~~~~~~
 
 Before working on a patch, you must determine on which branch you need to
-work. The branch should be based on the ``master`` branch if you want to add a
-new feature. But if you want to fix a bug, use the oldest but still maintained
-version of Symfony where the bug happens (like ``2.3``).
+work:
+
+* ``2.3``, if you are fixing a bug for an existing feature (you may have
+  to choose a higher branch if the feature you are fixing was introduced
+  in a later version);
+* ``2.7``, if you are adding a new feature which is backward compatible;
+* ``master``, if you are adding a new and backward incompatible feature.
 
 .. note::