update contribution guide for 2.7/3.0

Author: xabbuh

contributing/code/patches.rst

@@ -105,9 +105,11 @@ 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. The branch should be based on the ``2.7`` branch if you want to add
+a new feature that is backward compatible. If the new feature is not backward
+compatible, your branch should be based on the ``master`` branch. But if you
+want to fix a bug, use the oldest but still maintained version of Symfony
+where the bug happens (like ``2.3``).
 
 .. note::