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>