{readme.md} - use new style of loading a plugin

Author: saeideng

readme.md

@@ -27,8 +27,15 @@ php composer.phar require cakephp/authentication
 ```
 
 Load the plugin by adding the following statement in your project's
-`config/bootstrap.php`:
-
+`src/Application.php`:
+```php
+public function bootstrap()
+{
+    parent::bootstrap();
+    $this->addPlugin('Authentication');
+}
+```
+Prior to 3.6.0
 ```php
 Plugin::load('Authentication');
 ```