Update installation.rst

Adding notice that the AcmeDemoBundle is only accessible in the dev environment. So that newbies are less likely to see an Exception right from the start.
Explain why dev environement is necessary

Author: falk-graeser

book/installation.rst

@@ -128,6 +128,20 @@ And then access the CMF via:
 
     http://localhost:8000
 
+
+Or if you run a default apache installation like described in the cookbook of symfony (http://symfony.com/doc/current/cookbook/configuration/web_server_configuration.html):
+
+.. code-block:: text
+
+    http://localhost/app_dev.php
+    
+.. note::
+
+    Adding the ``app_dev.php`` to the url in your browser is important to actually 
+    see the test page. Because the AcmeDemoBundle is only configured to work with the 
+    Development Environment. (If you have a look at ``AppKernel.php`` you can easily
+    spot why)
+
 .. sidebar:: Using Other Database Backends
 
     If you prefer to use another database backend, for example MySQL, run the