[@themerdev/vim] add neovim instructions

[Deleted user]

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