Add the ability to format to devshell (and/or documenting it)

[Ben-PH]

I find myself using nix shell nixpkgs#rustfmt -c cargo fmt to format my code base. Either it's missing from the project config, or it's there, but there's a different way to do it that I'm not aware.

[a-kenji]

Yes, there is a different way, I will document it in CONTRIBUTING.md, basically there are two ways:

treefmt # in the devshell

or

nix fmt

[a-kenji]

Closed through: #170