Document FAQ for forge moderators and operators

From IRC:

<p​abs3> can someone respond to add-forge-now #123 with info about how often forges are scanned?
…
<p​abs3> vlorentz, olasd: both scanning period and repo refresh period would be interesting, preferably documented in a FAQ we can point folks to
<p​abs3> and also about how fast the repos are initially downloaded etc

Sounds like something we should have! :)