Teach SubmittingPatches about git-commit -s

As discussed on git mailing list let's teach the reader about
the possiblity to have automatically signed off the commit running
the git-commit -s command
Signed-off-by: NPaolo Ciarrocchi <paolo.ciarrocchi@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/SubmittingPatches

@@ -149,6 +149,9 @@ then you just add a line saying
 
 	Signed-off-by: Random J Developer <random@developer.example.org>
 
+This line can be automatically added by git if you run the git-commit
+command with the -s option.
+
 Some people also put extra tags at the end.  They'll just be ignored for
 now, but you can do this to mark internal company procedures or just
 point out some special detail about the sign-off.