Documentation: mention email-clients.txt in SubmittingPatches

I was struggling to get my email-client no to mangle my patch files,
and I didn't find enough information in the SubmittingPatches file.
By looking for more information on the web, I eventually found the
email-clients.txt file, and it answered all my needs

This patch adds a reference to email-clients.txt in SubmittingPatches,
and Mozilla related information which is no longer accurate
(as opposed to the details found in email-clients.txt).

This should be helpful for people sending their first patches,
or not sending patches on a frequent basis.

Signed-off-by: Michael Opdenacker <michael@free-electrons.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>