Add part about resolving parameter values to prepend extension

Author: alexander-schranz

bundles/prepend_extension.rst

@@ -82,6 +82,9 @@ in case a specific other bundle is not registered::
 
         // process the configuration of AcmeHelloExtension
         $configs = $container->getExtensionConfig($this->getAlias());
+        // resolve config parameters e.g. %kernel.debug% to boolean value
+        $resolvingBag = $container->getParameterBag();
+        $configs = $resolvingBag->resolveValue($configs);
         // use the Configuration class to generate a config array with
         // the settings "acme_hello"
         $config = $this->processConfiguration(new Configuration(), $configs);