Document how someone is supposed to build command-server

[fidgetingbits]

See the discussion here for context: talonhub/community#1362 (comment)

I used the nix flake PR to build, so I'm not actually sure how you build it yourself, but would be good to have an example for people who aren't using nix and to know the installation dependencies, etc.

[saidelike]

Fwiw I documented how to build the neovim's command-server in https://github.com/hands-free-vim/command-server/blob/a08bf9d8dad4e5fcfb5d20e83b6f655a2481a07f/README.md?plain=1#L23