Add a note on how root_path should be used

This follows WSGI, as root_path is considered the equivalent to
SCRIPT_NAME and ensures that the root_path can be used as a global
prefix to all routes in the application.

specs/www.rst

@@ -414,6 +414,17 @@ with the close code passed in the message (or 1000 if none is specified).
   spec.  Optional; if missing defaults to ``1000``.
 
 
+Path, raw_path and root_path
+----------------------------
+
+The ``root_path``, defaulting to ``""``, should match the start of any
+request target, if it doesn't the server should respond with a 404
+response. The ``path`` should then be constructed as the ``raw_path``
+excluding the ``root_path`` prefix. In the case where the resulting
+``path`` is equal to the empty string, ``""``, it should be set to
+``"/"``.
+
+
 WSGI Compatibility
 ------------------