Adding <stubs> for env() and param() (#297)

Info is taken from #292 (comment)

README.md

@@ -59,7 +59,7 @@ This file path may change based on your Symfony version, file structure and envi
 Default files are:
 - Symfony 3: `var/cache/dev/srcDevDebugProjectContainer.xml`
 - Symfony 4: `var/cache/dev/srcApp_KernelDevDebugContainer.xml`
-- Symfony 5: `var/cache/dev/App_KernelDevDebugContainer.xml`
+- Symfony 5+: `var/cache/dev/App_KernelDevDebugContainer.xml`
 
 Multiple container files can be configured. In this case, the first valid file is taken into account.
 If none of the given files is valid, a configuration exception is thrown.
@@ -80,6 +80,14 @@ If you're using PHP config files for Symfony 5.3+, you also need this for auto-l
 </extraFiles>
 ```
 
+If you're using Symfony's `env()` or `param()` functions in your PHP config files, you also need this for auto-loading them:
+
+```xml
+<stubs>
+    <file name="vendor/symfony/dependency-injection/Loader/Configurator/ContainerConfigurator.php" />
+</stubs>
+```
+
 If you're getting the following error
 
 > MissingFile - config/preload.php - Cannot find file ...var/cache/prod/App_KernelProdContainer.preload.php to include