minor #4070 Added a note about permissions in the Quick Tour (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added a note about permissions in the Quick Tour

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #4062

Commits
-------

141cf5e Added the missing title for a reference
bed2602 Minor rewording and fixed one reference
93b457e Fixed a wrong documentation reference (thanks @wouterj)
646a00c Added a note about permissions in the Quick Tour

Author: weaverryan

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 page or an error page.
+    This is caused by a directory permission misconfiguration. There are several
+    possible solutions depending on your operating system. All of them are
+    explained in the :ref:`Setting up Permissions <book-installation-permissions>`
+    section 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