Stable branches process documentation

- Added documentation for process about stable branches.

Change-Id: I84fd333641ae18df28638a6954dfa04ccce2bc24
diff --git a/Documentation/dev-contributing.txt b/Documentation/dev-contributing.txt
index 065e9d1..ca66968 100644
--- a/Documentation/dev-contributing.txt
+++ b/Documentation/dev-contributing.txt
@@ -238,6 +238,17 @@
     and it also makes "git revert" more useful.
   * Use topic branches to link your separate changes together.
 
+Process
+-------
+
+Backporting to stable branches
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+From time to time bug fix releases are made for existing stable branches.
+
+Developers concerned with stable branches are encouraged to backport or push
+patchsets to these branches, even if no new release is planned.
+
 
 GERRIT
 ------