Document how to get Kata GitHub review privs

[jodh-intel]

We need a document that explains how you get GitHub privs to allow you to "ack" PRs.

The current process is (too) informal and handled on the public ML and tends to involve an existing community member proposing team membersip privs for new users.

The doc needs to be referenced from -- or included as part of -- https://github.com/kata-containers/community/blob/master/CONTRIBUTING.md.

It should also suggest users granted privs consider "volunteering" for the PR and issue rota.