Add NPAPI deprecation message to extensions docs.

chrome/common/extensions/docs/templates/articles/npapi.html

 <h1>NPAPI Plugins</h1>
 
 <p>
 Leveraging HTML and JavaScript
 makes developing new extensions really easy,
 but what if you have existing legacy or proprietary code
 that you want to reuse in your extension?
 You can bundle an NPAPI plugin with your extension,
 allowing you to call into native binary code from JavaScript.
 </p>
 
 <h2 id="warning">Warning</h2>
 
+<p align="center"><b><a href="http://blog.chromium.org/2013/09/saying-goodbye-to-our-old-friend-npapi.html">NPAPI is being phased out.</a>
+Consider using alternatives.</b></p>
+
 <p align="center"><b>NPAPI is a really big hammer that should only be used when no other approach will work.</b>
 
 <p>Code running in an NPAPI plugin has the full permissions of the current user and is not sandboxed or shielded from malicious input by Google Chrome in any way. You should be especially cautious when processing input from untrusted sources, such as when working with <a href="content_scripts.html#security-considerations">content scripts</a> or XMLHttpRequest.
 
 <p>Because of the additional security risks NPAPI poses to users, extensions that use it will require manual review before being accepted in the
 <a href="https://chrome.google.com/webstore">Chrome Web Store</a>.</p>
 
 <h2 id="details">Details</h2>
 
 <p>
@@ skipping 67 matching lines @@
 to install malicious software on the user's machine.  Instead, avoid
 including an NPAPI plugin whenever possible.
 </p>
 
 <p>
 Marking your NPAPI plugin "public" increase the attack surface of your
 extension because the plugin is exposed directly to web content, making
 it easier for a malicious web site to manipulate your plugin.  Instead,
 avoid making your NPAPI plugin public whenever possible.
 </p>