Documentation for transport layer implementors

[Veritius]

There needs to be better documentation for people trying to write transport layers. Most of it can be more or less figured out from digging through rustdoc, but high level stuff would be good.

Also, the NetworkGroup component is hidden away in a tiny module which is hard to find. It's a pretty big issue if users expect to be able to send to a NetworkGroup but a transport layer doesn't do anything.