List port and host used in README

[kuon]

To allow easier firewall configuration, it would make sense to have a section the README detailing the traffic generated by tandem.

Like:

• TCP connection to rendezvous.foo.com on port 443 using HTTPS
• UDP connection to the either the remote host on a dynamic port or the rendezvous server

Also, what happen if the two hosts are on different local subnets that are directly connected, is a direct local connection tried first?