Add target branch notes

doc/contributing.rst

@@ -14,7 +14,7 @@ steps.
 
 * Optionally, add some technical documentation.
 
-* Send a pull request. Bonus points for topic branches.
+* Send a pull request, to the correct `target branch`_. Bonus points for topic branches.
 
 If you have a big change or would like to discuss something,
 please join us on the `mailing list
@@ -25,6 +25,22 @@ please join us on the `mailing list
     Any code you contribute must be licensed under the MIT
     License.
 
+Target branch
+=============
+
+Before you create a pull request for Silex, you need to determine which branch
+to submit it to. Read this section carefully first.
+
+Silex has two active branches: `1.0` and `master` (`1.1`).
+
+* **1.0**: Bugfixes and documentation fixes go into the 1.0 branch. 1.0 is
+  periodically merged into master. The 1.0 branch targets versions 2.1, 2.2 and
+  2.3 of Symfony2.
+
+* **1.1**: All new features go into the 1.1 branch. Changes cannot break
+  backward compatibility. The 1.1 branch targets the 2.3 version of Symfony2.
+
+
 Writing Documentation
 =====================