README: document NixOS setup

[nim65s]

Hi,

Following discussions between the ergonautes stand and the nix workshop attendees at Capitole du Libre this weekend, NixOS/nixpkgs#386205 was finally merged 🎉

Thanks @picnoir !
(and good luck for your bépo -> ergo-L transition :D)

I'm personally a happy user of something like:

services.arsenik = {
  enable = true;
  long_hold_timeout = 180;
  lt = true;
  vim = true;
  package = pkgs.arsenik.overrideAttrs {
    patches = [
      (pkgs.fetchpatch {
        name = "kanata-numpad-add-operators.patch";
        url = "https://github.com/OneDeadKey/arsenik/pull/77.patch";
        hash = "sha256-fIGp9IZ19gUq5tsnuX4I2KOLBA7HaF6u1berDleSnKg=";
      })
    ];
  };
};

(I'll leave this PR in draft until this nixos-unstable branch is reached, ref. https://nixpk.gs/pr-tracker.html?pr=386205, because the link is not yet available)