Update CONTRIBUTING with commit message tips

CONTRIBUTING.adoc

@@ -33,6 +33,9 @@ added after the original pull request but before a merge.
 * A few unit tests would help a lot as well -- someone has to do it.
 * If no-one else is using your branch, please rebase it against the current master (or
   other target branch in the main project).
+* When writing a commit message please follow http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html[these conventions],
+  if you are fixing an existing issue please add `Fixes gh-XXXX` at the end of the commit
+  message (where XXXX is the issue number).
 
 == Working with the code
 If you don't have an IDE preference we would recommend that you use