[WIP] Initial documentation of nvim features

Starting some documentation so users read about Neovim from within nvim!
@tarruda  Shouldn't this be `rpcsend` in analogy to `jobsend`? (Is `jobsend` blocking or does it return immediately?)

This would also comport with win32's distinction between `SendMessage` (synchronous) vs `PostMessage` (async): http://stackoverflow.com/a/3376637/152142