Add Documentation for NixOS

I packaged psd for NixOS:
https://github.com/NixOS/nixpkgs/pull/5316

Instead of configuring `/etc/psd.conf`, users just modify their system's
`configuration.nix` file to enable and configure the `psd` service:
```nix
{ # ...
  services.psd = {
    enable = true;
    users = [ "user42" "other_user" ];
    browsers = [ "firefox" "chromium" ];
    resyncTimer = "30m";
  };
}
```

Then rebuild the system with `nixos-rebuild switch`, and everything else will
be automatically installed and setup.

v5.53 is currently available in the `nixos-unstable` channel and 5.54 will be coming to
trunk(`nixpkgs/master`) soon.