docs: add sample code for Acme\Blog namespace definition

Author: kenjis

user_guide_src/source/general/modules.rst

@@ -101,7 +101,17 @@ This is configured in the file **app/Config/Modules.php**.
 The auto-discovery system works by scanning for particular directories and files within psr4 namespaces that have been defined in **Config/Autoload.php**.
 
 To make auto-discovery work for our **Blog** namespace, we need to make one small adjustment.
-**Acme** needs to be changed to **Acme\\Blog** because each "module" within the namespace needs to be fully defined. Once your module folder path is defined, the discovery process would look for discoverable items on that path and should, for example, find the routes file at **/acme/Blog/Config/Routes.php**.
+**Acme** needs to be changed to **Acme\\Blog** because each "module" within the namespace needs to be fully defined.
+
+::
+
+    public $psr4 = [
+        APP_NAMESPACE => APPPATH, // For custom namespace
+        'Config'      => APPPATH . 'Config',
+        'Acme\Blog'   => ROOTPATH . 'acme/Blog', // Change
+    ];
+
+Once your module folder path is defined, the discovery process would look for discoverable items on that path and should, for example, find the routes file at **/acme/Blog/Config/Routes.php**.
 
 Enable/Disable Discover
 =======================