update README with router.params promise info

README.md

@@ -115,6 +115,10 @@ Maps the specified path parameter `name` to a specialized param-capturing middle
 
 This function positions the middleware in the same stack as `.use`.
 
+If a `Promise` object is returned from the `param_middleware` function, the router
+will attach an `onRejected` callback using `.then`. If the promise is rejected,
+`next` will be called with the rejected value, or an error if the value is falsy.
+
 Parameter mapping is used to provide pre-conditions to routes
 which use normalized placeholders. For example a _:user_id_ parameter
 could automatically load a user's information from the database without