SubmittingPatches: document how to retract a topic

While this document outlines an idealized lifecycle where an author
develops a patch, refines it with reviewer feedback, and
successfully merges it into Git, reality is rarely so seamless.

Sometimes, a topic must be abandoned. Doing so explicitly is far
better than leaving it in limbo, especially since topics can always
be resurrected later.

Clearly state that we encourage contributors to retract any topic
that does not pan out.

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

Documentation/SubmittingPatches

@@ -95,6 +95,20 @@ input and avoids unnecessary churn from many rapid iterations.
   top, it gets merged to the 'master' branch and waits to become part
   of the next major release.
 
+But sometimes things do not work as planned:
+
+. If a mailing list discussion convinces you that your changes aren't
+  ideal, please explicitly retract the topic to save the maintainer
+  time and effort.
+
+. If you must drop a topic due to shifting priorities, lack of time,
+  or other commitments, notify the list as a courtesy so others can
+  take over. Anyone can resurrect the topic later when they have the
+  capacity to do so.
+
+. Topics with unaddressed review comments that remain inactive for
+  four weeks may be discarded by the maintainer.
+
 In the following sections, many techniques and conventions are listed
 to help your patches get reviewed effectively in such a life cycle.