SubmittingPatches: document how to reference previous commits

To reference previous commits people used to put just the
abbreviated SHA-1 into commit messages.  This is what has evolved as
a more stable format for referencing commits.  So lets document it
for everyone to look-up when needed.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -121,6 +121,11 @@ its behaviour.  Try to make sure your explanation can be understood
 without external resources. Instead of giving a URL to a mailing list
 archive, summarize the relevant points of the discussion.
 
+If you want to reference a previous commit in the history of a stable
+branch use the format "abbreviated sha1 (subject, date)". So for example
+like this: "Commit f86a374 (pack-bitmap.c: fix a memleak, 2015-03-30)
+noticed [...]".
+
 
 (3) Generate your patch using Git tools out of your commits.