[WIP] Initial documentation of nvim features

Starting some documentation so users read about Neovim from within nvim!
`jobsend` is non-blocking, so it is somewhat equivalent to `rpcnotify`, except that `rpcnotify` can be sent to all connected channels.

Though I'd rather leave the similarities contained in the `start`/`stop` functions because `notification` and `request` are part of msgpack-rpc, so it will be more intuitive to those that read the specification.