SubmittingPatches: release-notes entry experiment

SubmittingPatches: release-notes entry experiment

The "What's cooking" report lists the topics in flight, with a short
paragraph descibing what they are about.

Once written, the description is automatically picked up from the
"What's cooking" report and used in the commit log message of the
merge commit when the topic is merged into integration branches.
These commit log messges of the merge commits are then propagated to
the release notes.

It has been the maintainer's task to prepare these entries in the
"What's cooking" report.  Even though the original author of a topic
may be in the best position to write the initial description of a
topic, we so far lacked a formal channel for the author to suggest
what description to use.  The usual procedure has been for the
author to see the topic described in "What's cooking" report, and
then either complain about inaccurate explanation and/or offer a
rewrite.

Let's try an experiment to optionally let the author propose the one
paragraph description when the topic is submitted.  Pick the cover
letter as the logical place to do so, and describe an experimental
workflow in the SubmittingPatches document.

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