Update the_big_picture.rst

When following the steps in this documentation, I ended up with this: http://i.imgur.com/nK3XM2B.png
Got the suggestion to install the assets on the IRC channel and it did the trick for me. This addition to the docs might help others facing the same problem to make their app look as expected.

Author: oldskool

quick_tour/the_big_picture.rst

@@ -106,6 +106,13 @@ Congratulations! Your first Symfony project is up and running!
     them are explained in the
     :ref:`Setting up Permissions <book-installation-permissions>` section
     of the official book.
+    
+    If the welcome page does not seem to be rendering CSS or image assets,
+    install them first:
+    
+    .. code-block:: bash
+    
+        $ php app/console assets:install
 
 When you are finished working on your Symfony application, you can stop
 the server with the ``server:stop`` command: