refactored nixos directory & updated README

nixos/display-manager.nix

@@ -0,0 +1,18 @@
+{ pkgs, ... }:
+
+{
+  # Enable Display Manager
+  services.greetd = {
+    enable = true;
+    settings = {
+      default_session = {
+        command = "${pkgs.greetd.tuigreet}/bin/tuigreet --time --time-format '%I:%M %p | %a • %h | %F' --cmd Hyprland";
+        user = "greeter";
+      };
+    };
+  };
+
+  environment.systemPackages = with pkgs; [
+    greetd.tuigreet
+  ];
+}