SubmittingPatches: provide tag naming advice

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>