Add documentation deprecation notice and link to Web Fundamentals

As discussed with Paul, adding a quick deprecation notice as this is still the top result for 'Add to home screen' queries

android/installtohomescreen.html

@@ -11,7 +11,10 @@ dd { margin: 0 6px 12px 40px; }
 
 <div style="width:500px">
 
-<p style="margin-top:50px">Since Chrome 31,
+<p style="margin-top:50px"><b>Update: this documentation is now out of date. for up to date information
+please see the <a href="https://developers.google.com/web/fundamentals/engage-and-retain/app-install-banners/">
+Web App Install Banners documentation on Web Fundamentals</a>.</b></p>
+<p>Since Chrome 31,
 you can set up your web app to have an application shortcut icon added to a device's homescreen,
 and have the app launch in full-screen "app mode" using Chrome for Android’s "Add to homescreen" menu item.</p>