Write Usage documentation

[gilligan]

The README now has an API section, the next step is to add a Usage segment which should also cover overrides, the different ways to override (with defaults, without, with modified overrides, without any overrides) and mention the possibility to use preferWheel

[jtrakk]

I've also been wondering how to install it.

[surajbarkale]

Here is a shell.nix example https://github.com/mjlbach/nix-shells/blob/master/lorri_tensorflow_nixgl_poetry2nix/shell.nix