docs: Add optional dependencies to getting started guide.

Many users have had some issues with getting things to work
correctly because they did not realize that certain dependencies
were optional and not automatically included.

Add a section to the getting started guide to let people know
which dependencies these are.

Fixes #2820. References #4921. Closes #5123.

docs/app/partials/getting-started.tmpl.html

@@ -18,6 +18,22 @@ <h3>Including Angular Material and its dependencies</h3>
       <li><a href="https://github.com/angular/material#cdn">Using a CDN</a> (example below)</li>
     </ul>
 
+    <h3>Include Optional Dependencies</h3>
+    <p>
+      Angular Material integrates with some additional libraries which you may optionally include:
+    </p>
+    <ul style="margin-bottom: 2em;">
+      <li>
+        <a href="https://docs.angularjs.org/api/ngMessages">ngMessages</a>
+        - Provides a consistent mechanism for displaying form errors and other messages.
+      </li>
+
+      <li>
+        <a href="https://docs.angularjs.org/api/ngRoute">ngRoute</a>
+        - Provides a clean routing system for your application.
+      </li>
+    </ul>
+
     <iframe height='272' scrolling='no' data-default-tab="html"
             src='//codepen.io/marcysutton/embed/OPbpKm?height=272&theme-id=11083'
             frameborder='no' allowtransparency='true' allowfullscreen='true' style='width: 100%;'>