Move comments to the code

[teto]

I feel similarly to https://discourse.nixos.org/t/haumea-filesystem-based-module-system-for-nix/26902/15, aka I have trouble making sense of the documentation. I find the readme great but the git book useless. I had to look at @figsoda 's dotfiles to see how to use it.

I cloned the repo to get a better understanding and was surprised to see that the comments exist in the doc book but not in the code. Moving the doc to src/load.nix the doc for the load function with what it expects as input would help greatly IMO.