add instructions for running nyx safely to the FAQ

On the tor-relay-debian page, we used to tell people how to configure arm/nyx safely: see item 13 on https://web.archive.org/web/20171019233402/https://www.torproject.org/docs/tor-relay-debian The trick is to add your user to the debian-tor group, not to sudo your nyx to run as the debian-tor user.

We seem to have dropped those instructions when we migrated to the wiki page at https://trac.torproject.org/projects/tor/wiki/TorRelayGuide

I noticed just now because I was trying to help another arm/nyx user on #tor, who was doing it the wrong way (presumably because they were following old instructions from somewhere else, like the old arm documentation).