update contribution guide for 2.7/3.0

Author: xabbuh

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::