Document --dry-run parameter to send-email.

Looks like --dry-run was added to the code, but never to the --help output.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-send-email.perl

@@ -77,6 +77,8 @@ Options:
    --quiet	  Make git-send-email less verbose.  One line per email
                   should be all that is output.
 
+   --dry-run	  Do everything except actually send the emails.
+
 EOT
 	exit(1);
 }