patman: add Commit-notes tag and section

Sometimes a commit should have notes enclosed with it rather
than withing the cover letter -- possibly even because there
is no cover letter. Add a 'Commit-notes' tag, similar to the
'Series-notes' one; lines between this tag and the next END
line are inserted in the patch right after the '---' commit
delimiter.

Change-Id: I01e99ae125607dc6dec08f3be8a5a0b37f0a483d
Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Simon Glass <sjg@chromium.org>
(Updated README)