Added config example for administrators option

docs/index.md

@@ -111,6 +111,7 @@ Once we have it installed, we have to configure it on your `config.php` file.
     'user' => [
         'class' => Da\User\Module::class,
         // ...other configs from here: [Configuration Options](installation/configuration-options.md), e.g.
+        // 'administrators' => ['admin'], // this is required for accessing administrative actions
         // 'generatePasswords' => true,
         // 'switchIdentitySessionKey' => 'myown_usuario_admin_user_key',
     ]