Thomas Gummerer authored May 30, 2018 and Junio C Hamano committed Jun 01, 2018

Add a mention of the security mailing list to the README, and to
Documentation/SubmittingPatches..  2caa7b8d

 ("git manpage: note
git-security@googlegroups.com", 2018-03-08) already added it to the
man page, but for developers either the README, or the documentation
on how to contribute (SubmittingPatches) may be the first place to
look.

Use the same wording as we already have on the git-scm.com website and
in the man page for the README, while the wording is adjusted in
SubmittingPatches to match the surrounding document better.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>