git manpage: note git-security@googlegroups.com

Add a mention of the security mailing list to the "Reporting Bugs"
section. There's a mention of this list at
https://git-scm.com/community but none in git.git itself.

The copy is pasted from the git-scm.com website. Let's use the same
wording in both places.

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

Documentation/git.txt

@@ -839,6 +839,9 @@ Report bugs to the Git mailing list <git@vger.kernel.org> where the
 development and maintenance is primarily done.  You do not have to be
 subscribed to the list to send a message there.
 
+Issues which are security relevant should be disclosed privately to
+the Git Security mailing list <git-security@googlegroups.com>.
+
 SEE ALSO
 --------
 linkgit:gittutorial[7], linkgit:gittutorial-2[7],