Mention that the `ISP` setting is mandatory

Because that's the truth :-)

README.md

@@ -24,34 +24,13 @@ For development, install `dev.txt` instead:
 Set up configuration
 ====================
 
-You may want to create and edit configuration file
-`wifiwithme/settings/local.py` (no setting is mandatory at the
-moment):
+Then create and edit a `wifiwithme/settings/local.py` configuration file
+`wifiwithme/settings/local.py`.
 
-URL Prefix
-----------
+Mandatory settings
+------------------
 
-Optionaly, you can define an url prefix (ex: `/foo/`) so that wifi-with-me is
-accessible under *http://example.com/foo/* :
-
-    URL_PREFIX='foo/'
-
-Notifications
--------------
-
-If you to receive notifications on each new contrib, customize those :
-
-List of notification recipients:
-
-    NOTIFICATION_EMAILS=['admin@example.tld']
-
-Notification sender address:
-
-    DEFAULT_FROM_EMAIL='notifier@example.tld'
-
-The wifi-with-me website URL (for links included in emails :)
-
-    SITE_URL="http://example.tld"
+You **must** Define some details about your ISP in the ISP variable, eg:
 
     ISP={
         'NAME':'FAIMAISON',
@@ -70,6 +49,35 @@ The wifi-with-me website URL (for links included in emails :)
         }
     }
 
+Optional settings
+-----------------
+
+### Website URL
+
+The wifi-with-me website URL (for links included in emails :)
+
+    SITE_URL="http://example.tld"
+
+
+## URL Prefix
+
+Optionaly, you can define an url prefix (ex: `/foo/`) so that wifi-with-me is
+accessible under *http://example.com/foo/* :
+
+    URL_PREFIX='foo/'
+
+### Notifications
+
+If you to receive notifications on each new contrib, customize those :
+
+List of notification recipients:
+
+    NOTIFICATION_EMAILS=['admin@example.tld']
+
+Notification sender address:
+
+    DEFAULT_FROM_EMAIL='notifier@example.tld'
+
 Migrate from bottle version (optional)
 ======================================