MyFirstContribution: add avenues for getting help

MyFirstContribution: add avenues for getting help

With https://lore.kernel.org/git/20191114194708.GD60198@google.com/ we
now have a mentoring mailing list, to which we should direct new
contributors who have questions.

Mention #git-devel, which is targeted for Git contributors; asking for
help with getting a first contribution together is on-topic for that
channel. Also mention some of the conventions in case folks are
unfamiliar with IRC.

Because the mentoring list and #git-devel are both a subset of Git
contributors, finally list the main Git list and mention some of the
posting conventions.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>