SubmittingPatches: add recommendation for mailing list references

SubmittingPatches already mentions referencing bugs fixed by a commit,
but doesn't mention citing relevant mailing list discussions.  Add a
note to that effect, along with a recommendation to use the
https://lkml.kernel.org/ redirector.

Portions based on text from git's SubmittingPatches.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Borislav Petkov <bp@suse.de>
Cc: Rob Landley <rob@landley.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>