added a note about committing patches to the 2.0 branch

Author: Burgov

contributing/code/patches.rst

@@ -154,6 +154,11 @@ Create the topic branch with the following command:
 
     $ git checkout -b BRANCH_NAME master
 
+.. tip::
+
+    Replace "master" by, for example, 2.0, if you wish to submit a patch to the
+    2.0 branch.
+
 .. tip::
 
     Use a descriptive name for your branch (`ticket_XXX` where `XXX` is the