SubmittingPatches: provide tag naming advice

Current statistics show a strong preference to only capitalize the first
letter in a hyphenated tag, but that some guidance would be helpful:

git log |
  perl -ne 'next unless /^\s+(?:Signed-[oO]ff|Acked)-[bB]y:/;
    s/^\s+//;s/:.*/:/;print'|
  sort|uniq -c|sort -n
   2 Signed-off-By:
   4 Signed-Off-by:
  22 Acked-By:
  47 Signed-Off-By:
2202 Acked-by:
95315 Signed-off-by:

Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -368,6 +368,9 @@ While you can also create your own trailer if the situation warrants it, we
 encourage you to instead use one of the common trailers in this project
 highlighted above.
 
+Only capitalize the very first letter of tags, i.e. favor
+"Signed-off-by" over "Signed-Off-By" and "Acked-by:" over "Acked-By".
+
 [[git-tools]]
 === Generate your patch using Git tools out of your commits.