Update README.md

Author: z-song

README.md

@@ -24,6 +24,30 @@ $ php artisan admin:import api-tester
 
 Finally open `http://localhost/admin/api-tester`.
 
+## Configuration
+
+`api-tester` supports 3 configuration, open `config/admin.php` find `extensions`:
+```php
+
+    'extensions' => [
+    
+        'api-tester' => [
+        
+            // route prefix for APIs
+            'prefix' => 'api',
+
+            // auth guard for api
+            'guard'  => 'api',
+
+            // If you are not using the default user model as the authentication model, set it up
+            'user_retriever' => function ($id) {
+                return \App\User::find($id);
+            },
+        ]
+    ]
+
+```
+
 License
 ------------
 Licensed under [The MIT License (MIT)](LICENSE).