[@themerdev/vim] add neovim instructions
## Problem

I'm confused on how to use Themer themes on NeoVim, since it does not work when adding it to `~/.config/nvim` or `~/.vim/colors`

## Solution

Add instructions to the generated `README.md`

## Alternatives

It would be confusing not to put it in the `README.md`

## Additional context

Neovim instructions are not in the `README.md`
