Documentation: Add note on sending files directly with Mutt

Like 'git send-email', Mutt can also be used to send patches generated
with 'git format-patch'. This works regardless of the editor the
contributor has set up to use with Mutt.

Signed-off-by: Eddie Kovsky <ewk@edkovsky.org>
Reviewed-by: Darren Hart <dvhart@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>