SubmittingPatches: encourage users to use format-patch and send-email

In step "(4) Sending your patches", we instruct users to do an
inline patch, avoid breaking whitespaces, avoid attachments, use
[PATCH v2] for second round, etc., all of which format-patch and
send-email combo know how to do well.

The need was identified by, and the text is based on the work by
Cody Taylor.
Suggested-by: NCody Taylor <cody.taylor@maternityneighborhood.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -135,6 +135,11 @@ that is fine, but please mark it as such.
 
 (4) Sending your patches.
 
+Learn to use format-patch and send-email if possible.  These commands
+are optimized for the workflow of sending patches, avoiding many ways
+your existing e-mail client that is optimized for "multipart/*" mime
+type e-mails to corrupt and render your patches unusable.
+
 People on the Git mailing list need to be able to read and
 comment on the changes you are submitting.  It is important for
 a developer to be able to "quote" your changes, using standard