Documentation: Add "how to write a good patch summary" to SubmittingPatches

Unfortunately many patch submissions are arriving with painfully poor
patch descriptions.   As a result of the discussion on LKML:

      http://lkml.org/lkml/2009/4/15/296

explain how to submit a better patch description, in the (perhaps
vain) hope that maintainers won't end up having to rewrite the git
commit logs as often as they do today.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>