Added a note about permissions in the Quick Tour

Author: javiereguiluz

quick_tour/the_big_picture.rst

@@ -89,6 +89,14 @@ main concepts. Go to the following URL to be greeted by Symfony2 (replace
 .. image:: /images/quick_tour/hello_fabien.png
    :align: center
 
+.. note::
+
+    Instead of the greeting page, you may see a blank or error page. This is
+    caused by a directory permission misconfiguration. There are several possible
+    solutions depending on your operating system. Check out them all in the
+    :doc:`installation chapter </book/installation.html#book-installation-permissions>`
+    of the official book.
+
 What's going on here? Have a look at each part of the URL:
 
 * ``app_dev.php``: This is a :term:`front controller`. It is the unique entry