Adding scanDirectories to README for Symfony 5.3+

Author: ThomasLandauer

README.md

@@ -66,6 +66,9 @@ parameters:
         container_xml_path: var/cache/dev/srcApp_KernelDevDebugContainer.xml
         # or with Symfony 5+
         container_xml_path: var/cache/dev/App_KernelDevDebugContainer.xml
+    # If you're using PHP config files for Symfony 5.3+, you also need this for auto-loading of `Symfony\Config`:
+    scanDirectories:
+        - var/cache/dev/Symfony/Config
 ```
 
 ## Constant hassers