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>