explicit source of variables in overview.md

Explicit source of Router, Route, DefaultRoute in overview.md

I've only see one person run into this on StackOverflow, but it doesn't hurt to be explicit (we are dealing with react users, right?).

Author: brigand

docs/guides/overview.md

@@ -144,6 +144,9 @@ Here's how it works:
    active child route.
 
 ```js
+var Router = require('react-router'); // or var Router = ReactRouter; in browsers
+var Route = Router.Route, DefaultRoute = Router.DefaultRoute;
+
 var App = React.createClass({
   render: function () {
     return (