[tor-mirrors] Updated mirror instructions

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.