Add neovim installation for the rust language

[eleijonmarck]

Summary

We have a vim binding but I would love to see the documentation and the setup for Neovim

Motivation

The idea is that neovim is booming more and more popular and especially since 0.5 where lsp is a battery included feature.

Drawbacks

Rationale and alternatives

Maintenance

Unresolved Questions

[AliSajid]

It is my understanding that neovim is practically a drop-in replacement for vim. At least that's how I've been using it. Given that, I believe just changing the tools page to say "Vim/NeoVim" with the link back to github for the rust plugin should be fine. I'll write a change and add a pull request.