Templates/default/modules/gnome: Add doc-comment - what the module does, how to use it

templates/clan/default/modules/gnome.nix

@@ -1,3 +1,17 @@
+/*
+  This is an example of a simple nixos module:
+
+  Enables the GNOME desktop environment and the GDM display manager.
+
+  To use this module, import it in your machines NixOS configuration like this:
+
+  ```nix
+  imports = [
+    modules/gnome.nix
+  ];
+  ```
+*/
+{ ... }:
 {
   services.xserver.enable = true;
   services.xserver.desktopManager.gnome.enable = true;