documentation

[qknight]

motivation

• it is not clear what carnix actually does and what not. if one has a 'clue' it still is hard to use as there is a 'outdated' manual https://nixos.org/nixpkgs/manual/#compiling-rust-crates-using-nix-instead-of-cargo and carnix --help shows completetly different usages
• the README.md does not include any usage guide

shouldn't we at least have a README.md manual on how to use it?