Add "Hello world" to index.html

This avoids the default template rendering a completely blank page and
makes it easy for people to know straight away that things are
"working".

Fix gh-1165

index.html

@@ -22,6 +22,7 @@
         <![endif]-->
 
         <!-- Add your site or application content here -->
+        <p>Hello world! This is HTML5 Boilerplate.</p>
 
         <script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"></script>
         <script>window.jQuery || document.write('<script src="js/vendor/jquery-1.8.0.min.js"><\/script>')</script>