docs: add example module

examples/module.nix

@@ -0,0 +1,49 @@
+# https://nixos.wiki/wiki/NixOS_modules
+
+# This module can be used as follows:
+# {
+#   imports = [ ./module.nix ];
+#   services.example = {
+#     enable = true;
+#     message = "Hello :)";
+#   };
+# }
+
+{ lib, config, options, pkgs, modulesPath, ... }:
+with lib;
+let
+  # cfg is a typical convention.
+  cfg = config.services.example;
+in
+{
+  imports = [
+    # Paths to other modules.
+    # Compose this module out of smaller ones.
+  ];
+
+  options = {
+    # Option declarations.
+    # Declare what settings a user of this module module can set.
+    # Usually this includes an "enable" option to let a user of this module choose.
+
+    services.example = {
+      enable = mkEnableOption "example service";
+      message = mkOption {
+        type = types.str;
+        default = "Hello, world!";
+        example = "Hi!";
+        # A textual description of the option, in DocBook format.
+        description = ''
+          You can set any message you want here.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    # Option definitions.
+    # Define what other settings, services and resources should be active.
+    # These are depend on whether a user of this module chose to "enable" it using the "option" above.
+    # You also set options here for modules that you imported in "imports".
+  };
+}