Update README.md

[claeusdev]

I've been using this theme for a while now and I love it a lot. I've recently had to update my vim config to neovim and it's amazing new tooling so I thought it would be nice to have this as part of the docs.

Add some docs for using this colorscheme via lua-based neovim package managers. i.e. Lazy and Packer.

Let me know what you think.