[README] Volunteers: temporary URL change

[rikker]

While the PGT website is being set up on a new host over the next few
weeks, volunteers can most reliably access PGT Unbindery at this web
address:

http://pgt.bencrowder.net

(In fact, this was always the actual address, and gutenbergthai.org
was simply forwarding to it for the time being.)

Thanks