[app] Add link to use mock data in GH example

src/app/docs/app/app-contributors.mustache

@@ -19,7 +19,9 @@
   * Provide more of an intro to the apps organization. Expand on this in a section after the rendered example, but before the code.
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 <p>
-This example demonstrates the powerful navigation and view management features `Y.App` provides by creating an application to browse through the contributors of GitHub projects.
+This example demonstrates the powerful navigation and view management features `Y.App` provides by creating an application to browse through the contributors of GitHub projects.</p>
+
+<p><strong>Note:</strong> If the GitHub API is unavailable, you can <a href="app-contributors.html?mock=true">try this example with mock data</a>. The available users in the mock example are `yui` and `ericf`.
 </p>
 </div>