Explain how to supersede the new dynamic welcome page

The old, static welcome page instructed developers to delete the
public/index.html file and set a root route.  The new, dynamic welcome
page should tell developers (a) why they're seeing it when it doesn't
correspond to anything in the viewable app source and (b) that it can
be superseded with a root route.

railties/lib/rails/templates/rails/welcome/index.html.erb

@@ -223,6 +223,8 @@
             </li>
 
             <li>
+              <h2>Set up a root route to replace this page</h2>
+              <p>You're seeing this page because you're running in development mode and you haven't set a root route yet.</p>
               <p>Routes are set up in <span class="filename">config/routes.rb</span>.</p>
             </li>