Add missing dependencies in docs/overview.md.

Author: lazywei

docs/guides/overview.md

@@ -145,7 +145,8 @@ Here's how it works:
 
 ```js
 var Router = require('react-router'); // or var Router = ReactRouter; in browsers
-var Route = Router.Route, DefaultRoute = Router.DefaultRoute;
+var Route = Router.Route, DefaultRoute = Router.DefaultRoute,
+  Link=Router.Link, RouteHandler = Router.RouteHandler;
 
 var App = React.createClass({
   render: function () {