Mention that you need a config/ dir for MicroKernelTrait

Author: javiereguiluz

configuration/micro_kernel_trait.rst

@@ -70,6 +70,12 @@ Next, create an ``index.php`` file that defines the kernel class and runs it::
     $response->send();
     $kernel->terminate($request, $response);
 
+.. note::
+
+    In addition to the ``index.php`` file, you'll need to create a directory called
+    ``config/`` in your project (even if it's empty because you define the configuration
+    options inside the ``configureContainer()`` method).
+
 That's it! To test it, start the :doc:`Symfony Local Web Server
 </setup/symfony_server>`: