[WIP] Initial documentation of nvim features

Starting some documentation so users read about Neovim from within nvim!
Hm.. actually, why Nvim and not Neovim? To distinguish between Neovim (the project) and Nvim (the editor, as opposed to other components of the project, such as the Neovim Python client)? Should this repo be called `neovim/nvim`, then?