Document `nix` in the `README.md`

[reinhrst]

Claude:

I saw you added nix when updating hugo version. Would you be able to write a couple of lines on that (and how/if you need it for anything while developing) in the README.md?

(Holy grail would be to have a Dockerfile to replicate an exact build environment but that's probably too much work for too little gain :))

@DoomHammer:

I will at some point thanks for spotting that ah, you can create an Issue so we won't forget later on 😄