Make vpn an extra app

Documents how to enable it.

README.md

@@ -245,6 +245,26 @@ Then  make a copy of the template file (and customize it) at
 
 Good to go :-)
 
+Using optional apps
+-------------------
+
+Some apps are not enabled by default :
+
+- *coin.vpn*: Management of VPN subscription and credentials
+
+You can enable them using the `EXTRA_INSTALLED_APPS` setting. Ex:
+
+    EXTRA_INSTALLED_APPS = (
+        coin.vpn',
+    ]
+
+If you enable an extra-app after initial installation, make sure to sync database :
+
+    ./manage.py migrate
+
+*nb: extra apps are loaded after the builtin apps.*
+
+
 Settings
 ========
 

coin/settings.py

@@ -162,7 +162,6 @@ INSTALLED_APPS = (
     'coin.resources',
     'coin.reverse_dns',
     'coin.configuration',
-    'vpn',
     'coin.isp_database',
     'simple_dsl'
 )