doc/SubmittingPatches: show how to get a CLI commit summary

Amend the section which describes how to get a commit summary to show
how do to that with "git show", currently the documentation only shows
how to do that with gitk.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -134,8 +134,9 @@ with the subject enclosed in a pair of double-quotes, like this:
     noticed that ...
 
 The "Copy commit summary" command of gitk can be used to obtain this
-format.
+format, or this invocation of "git show":
 
+    git show -s --date=short --pretty='format:%h ("%s", %ad)' <commit>
 
 (3) Generate your patch using Git tools out of your commits.