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>

Documentation/SubmittingPatches

@@ -220,20 +220,8 @@ decreasing the likelihood of your MIME-attached change being accepted.
 Exception:  If your mailer is mangling patches then someone may ask
 you to re-send them using MIME.
 
-
-WARNING: Some mailers like Mozilla send your messages with
----- message header ----
-Content-Type: text/plain; charset=us-ascii; format=flowed
----- message header ----
-The problem is that "format=flowed" makes some of the mailers
-on receiving side to replace TABs with spaces and do similar
-changes. Thus the patches from you can look corrupted.
-
-To fix this just make your mozilla defaults/pref/mailnews.js file to look like:
-pref("mailnews.send_plaintext_flowed", false); // RFC 2646=======
-pref("mailnews.display.disable_format_flowed_support", true);
-
-
+See Documentation/email-clients.txt for hints about configuring
+your e-mail client so that it sends your patches untouched.
 
 8) E-mail size.