Add high level docs for the module system?

[matklad]

I’ve been trying to casually read the source code for the past few days, and, while in the small it is exceptionally readable (type signatures are :heart), I must confess that the larger picture eludes me.

That is, we define things which are to be “run” later by something else, but so far I’ve been missing that “main”.

I think what I am missing is the module system. Specifically, the module.run function. I looked at it, and reverse engineering its behavior is beyond my (admittedly limited) understanding of nix/NixOS.

I feel like this might be a stumbling block for many contributors, so documenting, in prose, how modules work, seems to be high impact!