.github: Enable repo-lockdown bot to refuse GitHub pull requests

.github: Enable repo-lockdown bot to refuse GitHub pull requests

Some GitHub users try to open pull requests against the GitHub
mirror. Unfortunate these get ignored until eventually someone
notices and closes the request.

Enable the 'Repo Lockdown' [*] 3rd party bot which can autorespond
to pull requests with a friendly comment, close the request, and
then lock it to prevent further comments.

[*] https://github.com/dessant/repo-lockdown

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20200406214125.18538-1-f4bug@amsat.org>
[AJB: s/fill/file/ and point at canonical qemu.org/contribute]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>