[tor-mirrors] Updated mirror instructions

[Andrew Lewman]

At the very bottom of
https://www.torproject.org/docs/running-a-mirror.html.en, I added some
new info about setting up a mirror.

"In order to add your mirror, please send a single, comma delimited line
of text based on this file to the mirrors list. Your mirror will then
be added manually if it passes availability testing and your provided
information is confirmed. Do not run your mirror behind a content delivery
network (such as Akamai, Cloudflare, Fastly, etc), as most of them block
access from countries where the mirror is needed the most."

Feedback, clarifications, etc are all welcome.

-- 
Andrew
pgp 0x6B4D6475