Add undocumented config params to README.

README.md

@@ -83,6 +83,10 @@ attributes:
     client IP address
   * `max_inflight_requests`: maximum number of in-flight requests to
     allow before server-side throttling kicks in
+ * `site_name`: sting to be used as site `<title>`.
+ * `site_logo`: path to an image to be used as logo, placed above the
+    modal form.
+ * `site_favicon`: path to a favicon.
 
 ## Device tracking