Documentation/SubmittingPatches: Instruct how to use [PATCH] Subject header

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -112,7 +112,12 @@ lose tabs that way if you are not careful.
 
 It is a common convention to prefix your subject line with
 [PATCH].  This lets people easily distinguish patches from other
-e-mail discussions.
+e-mail discussions.  Use of additional markers after PATCH and
+the closing bracket to mark the nature of the patch is also
+encouraged.  E.g. [PATCH/RFC] is often used when the patch is
+not ready to be applied but it is for discussion, [PATCH v2],
+[PATCH v3] etc. are often seen when you are sending an update to
+what you have previously sent.
 
 "git format-patch" command follows the best current practice to
 format the body of an e-mail message.  At the beginning of the