Document @rfcbot

[jyn514]

https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.20meeting.20followup.5D.202020-09-11.20.2354818/near/209797400

[ehuss]

There is a link to the rfcbot documentation here: https://forge.rust-lang.org/infra/service-infrastructure.html#rfcbot
That should appear as the first hit if you search for "rfcbot".