Provide list of TMate servers in documentation

[EricPoggel]

I've found this list by browsing through other issues on github. But this should be documented. For example today we needed to update the firewall rules at a client site to allow TMate to make outgoing connections to these servers.

134.209.128.51    nyc1.tmate.io   New York City
157.230.72.130    sfo2.tmate.io   San Francisco
188.166.207.127   sgp1.tmate.io   Singapore
206.189.246.93    lon1.tmate.io   London

The documentation should also explain that TMate will connect to these servers on outgoing port 22.