Add documentation on how to forward ports

Created by: mray

There needs to be a common indication of ports that need to be forwarded in case of running a FreedomBox behind a NAT.

For example: I installed the xmpp server - now what ports does it need, a quick search only talks about 5222, but there are lots of others that i might want to use for file transfer or ssl or whatever!

Ideally that would also always link to a resource that explains the procedure.

Original Title: Indicate port forwarding requirements of apps