refactored nixos directory & updated README

nixos/screen.nix

@@ -0,0 +1,19 @@
+{ pkgs, ... }:
+
+{
+  services.geoclue2.appConfig = {
+      "gammastep" = {
+        isAllowed = true;
+        isSystem = false;
+        users = [ "1000" ]; # FIXME: set your user id (to get user id use command 'id -u "your_user_name"')
+      };
+  };
+  
+  programs.light.enable = true;
+
+  environment.systemPackages = with pkgs; [
+    # wlsunset
+    gammastep
+    brightnessctl
+  ];
+}