SETUP
Users of NixOS MUST be tracking nixos-unstable; this will change once the next version (greater than 14.12) is finalized:

 nix-channel --add http://nixos.org/channels/nixos-unstable nixos
 nixos-rebuild switch --upgrade
The NixOS module looks for a services.psd attribute set in /etc/nixos/configuration.nix (run nixos-help for all options). As with any modification of this file, rebuild the system configuration with nixos-rebuild switch.

 services.psd = {
  enable = true;
        users = [ "facade" "debbie" ]; # At least one user is required
        browsers = [ "chromium" "firefox" ]; # Leave blank to enable all supported browsers
        useOverlayFS = true; # set to true to enable overlayfs or set to false to use the default sync mode
 };