openafs: add a contributor code of conduct

openafs: add a contributor code of conduct

In the interest of fostering a friendly, welcoming environment
for contributors, institute a code of conduct for the project.

Adapted from the Contributor Covenant.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/11987
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jonathan A. Kollasch <jakllsch@kollasch.net>
Reviewed-by: Thomas L. Kula <kula@tproa.net>
Reviewed-by: Nathaniel Filardo <nwfilardo@gmail.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit c6a8739a67edca8a7e987bd1f3d5dcc226ad47a0)

Change-Id: I797bd8256a8509e3b75c8d5cd708fa5edeab64c4
Reviewed-on: http://gerrit.openafs.org/12056
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>