Documentation/SubmittingPatches: discuss In-Reply-To

Add a paragraph suggesting best practices for when to link patches
to previous LKML messages via In-Reply-To.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
[jc: moved the added text to a separate section]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>