document how git.tpo->github mirroring happens
It seems like no one knows how the git.torproject.org mirroring of tor.git to github.com happens.
An account named "tor-bot" seems to be involved. Maybe someone with shell access to git.tpo should look at the hook scripts?
Also, there might be an intermediate repository involved, because a couple of deleted branches on git.tpo keep reappearing on github.com.
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