add a Code of Conduct document

add a Code of Conduct document

We've never had a formally written Code of Conduct document. Though it
has been discussed off and on over the years, for the most part the
behavior on the mailing list has been good enough that nobody felt the
need to push one forward.

However, even if there aren't specific problems now, it's a good idea to
have a document:

  - it puts everybody on the same page with respect to expectations.
    This might avoid poor behavior, but also makes it easier to handle
    it if it does happen.

  - it publicly advertises that good conduct is important to us and will
    be enforced, which may make some people more comfortable with
    joining our community

  - it may be a good time to cement our expectations when things are
    quiet, since it gives everybody some distance rather than focusing
    on a current contentious issue

This patch adapts the Contributor Covenant Code of Conduct. As opposed
to writing our own from scratch, this uses common and well-accepted
language, and strikes a good balance between illustrating expectations
and avoiding a laundry list of behaviors. It's also the same document
used by the Git for Windows project.

The text is taken mostly verbatim from:

  https://www.contributor-covenant.org/version/1/4/code-of-conduct.html

I also stole a very nice introductory paragraph from the Git for Windows
version of the file.

There are a few subtle points, though:

  - the document refers to "the project maintainers". For the code, we
    generally only consider there to be one maintainer: Junio C Hamano.
    But for dealing with community issues, it makes sense to involve
    more people to spread the responsibility. I've listed the project
    committee address of git@sfconservancy.org as the contact point.

  - the document mentions banning from the community, both in the intro
    paragraph and in "Our Responsibilities". The exact mechanism here is
    left vague. I can imagine it might start with social enforcement
    (not accepting patches, ignoring emails) and could escalate to
    technical measures if necessary (asking vger admins to block an
    address). It probably make sense _not_ to get too specific at this
    point, and deal with specifics as they come up.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: CB Bailey <cb@hashpling.org>
Acked-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Emily Shaffer <emilyshaffer@google.com>
Acked-by: Garima Singh <garimasigit@gmail.com>
Acked-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Jonathan Tan <jonathantanmy@google.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Acked-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: brian m. carlson <sandals@crustytoothpaste.net>
Acked-by: Derrick Stolee <stolee@gmail.com>
Acked-by: Thomas Gummerer <t.gummerer@gmail.com>
Acked-by: William Baker <williamtbakeremail@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>