Add documentation on the new configuration parameters

README.md

@@ -61,6 +61,10 @@ attributes:
     * `cert`: path to the client certificate
     * `key`: path to the private key
     * `ca`: path to the CA used to validate the server
+* `u2f_app_id`: set the U2F AppID - if unset, it will be autodetected
+  based on the domain name in the request
+* `url_path_prefix`: URL path prefix of the SSO server application
+  (default /)
 * `http_server`: specifies standard parameters for the HTTP server
   * `tls`: server-side TLS configuration
     * `cert`: path to the server certificate