gitlab: add CONTRIBUTING.rst file to indicate use of merge requests

With the introduction of automated CI pipelines, we are now ready to switch
to using merge requests for the project. With this switch we longer wish
to have patches sent to the mailing list, and thus the git-publish
config is removed.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>