README: add example that doesn't check out / live inside this repo

[tobiasvandriessel]

I think it'd make it easier to get started with this project if there were an example that doesn't need to check out or live inside this repo. For example, linking from the README to tstat's old example (although I would keep the changed values for the cachix config) would already be a great help. In the end I used that example and the flake inside this repo to get to a first draft version of a configuration; I think it would reduce the time to get started for new users if these were linked to directly.

Additionally, tstat's example illustrates the parts in the README on how to add the cachix configuration, and how to add the sd-image nixosModule to facilitate building the sd card.