Add rutorrent discovery?

Hey!

I've been thinking about how some people might configure rutorrent using a RPI or similar devices on their home and access it through port forward or ssh tunneling services like remote.it and I was wondering maybe we should add a discovery helper. Also, since it seems like rTorrent & rutorrent is very much user-configured, we could add some instructions to set a permanent announcement through Avahi xml files.

We would need to agree on:
* a service type
* a sample xml to announce the service through Avahi
* maybe some TXT records that might help the app discover the app correctly (maybe add TXT records to set ~the protocol you want to access~ if the server uses http or https, a public URL where this server might be accessed, or anything else, really).

I could write a sample PR that adds this discovery through the setup interface, but comments are welcome.