Doc SubmittingPatches: Mention --notes option after "cover letter"

The git format-patch --notes option can now insert the commit notes
after the three dashes. Mention this after the regular cover letter
guidance for submitting patches.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Jeff King <peff@peff.net>

Documentation/SubmittingPatches

@@ -179,7 +179,8 @@ message starts, you can put a "From: " line to name that person.
 
 You often want to add additional explanation about the patch,
 other than the commit message itself.  Place such "cover letter"
-material between the three dash lines and the diffstat.
+material between the three dash lines and the diffstat. Git-notes
+can also be inserted using the `--notes` option.
 
 Do not attach the patch as a MIME attachment, compressed or not.
 Do not let your e-mail client send quoted-printable.  Do not let