README: Better Neovim instructions, .luarc.json example

[Cynosphere]

• NeoVim -> Neovim
• Configuration URL now goes to nvim-lspconfig as the LuaLS README no longer includes instructions (the instructions on the site also link to here and nvim-lspconfig link back to the site as well)
• Rework Neovim setup instructions to create an addons folder that copies the VSCode extension's structure and to prefer setting addon paths per project in .luarc.json instead of globally
• Add an example .luarc.json

[luttje]

Thanks for contributing. Very clean PR making this easy to merge. I don't personally use Neovim so your help is greatly appreciated.

@all-contributors please add @Cynosphere for doc

[allcontributors]

@luttje

I've put up a pull request to add @Cynosphere! 🎉