refactored nixos directory & updated README

nixos/users.nix

@@ -0,0 +1,22 @@
+{ pkgs, ... }:
+
+{
+  # Define a user account. Don't forget to set a password with ‘passwd’.
+  users.users.xnm = {
+    isNormalUser = true;
+    description = "xnm";
+    extraGroups = [ "networkmanager" "input" "wheel" "video" "audio" "tss" ];
+    shell = pkgs.fish;
+    packages = with pkgs; [
+      spotify
+      youtube-music
+      discord
+      tdesktop
+      vscode
+      brave
+    ];
+  };
+
+  # Change runtime directory size
+  services.logind.extraConfig = "RuntimeDirectorySize=8G";
+}