add reference to underscore.js on docs page, since this is a good place to play around with it in the console

index.html

@@ -4,6 +4,7 @@
   <meta http-equiv="content-type" content="text/html;charset=UTF-8" />
   <meta http-equiv="X-UA-Compatible" content="chrome=1">
   <title>Underscore.js</title>
+  <script src="underscore.js"></script>
   <style>
     body {
       font-size: 16px;