Update overview.md

RouteHandler is mentioned but its not clear where to get it, add to commonjs guide.

Author: iammerrick

docs/guides/overview.md

@@ -433,4 +433,5 @@ var Route = Router.Route;
 var NotFoundRoute = Router.NotFoundRoute;
 var DefaultRoute = Router.DefaultRoute;
 var Link = Router.Link;
+var RouteHandler = Router.RouteHandler;
 ```