Add flake

[Morgenkaff]

Hi Agateau

Is we talked about on Mastodont; here is a PR to add the flake I wrote to the official repo. I've kept the files for the flake in the folder "nix".

Besides of the, have I added some lines in the README about the flake. Please edit anything you like, and say if I should change anything!

Best regards Morgenkaff

Edit: Typo