Accessibility: mention required Racket version and supply shell.nix

[Mr-Andersen]

Unfortunately, Hackett doesn't seem to work with Racket's two of latest major versions. To keep the project accessible to those who are interested, I've specified required Racket version and added shell.nix to simplify it's usage. No rebuilds needed, surprisingly! Even though this nixpkgs version is 6 years old, they seem to have kept all required dependencies in cache.

P.S. Still hope you'll return to this eventually :cry: