add sidebar for the built-in server in VMs

Author: xabbuh

cookbook/web_server/built_in.rst

@@ -36,6 +36,23 @@ can change the socket passing an ip address and a port as a command-line argumen
 
     $ php app/console server:run 192.168.0.1:8080
 
+.. sidebar:: Using the built-in Web Server from inside a Virtual Machine
+
+    If you want to use the built-in web server from inside a virtual machine
+    and then load the site from a browser on your host machine, you'll need
+    to listen on the ``0.0.0.0:8000`` address (i.e. on all ip addresses that
+    are assigned to the virtual machine):
+
+    .. code-block:: bash
+
+        $ php app/console server:run 0.0.0.0:8000
+
+    .. caution::
+
+        You should **NEVER** listen to all interfaces on a computer that is
+        directly accessible from the Internet. The built-in web server is
+        not designed to be used on public networks.
+
 Command Options
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