Adding cautionary note about manifest_version 2 and Chrome 17.

chrome/common/extensions/docs/static/manifestVersion.html

 <div id="pageData-name" class="pageData">Manifest Version</div>
 
 <p>
   Extensions, themes, and applications are simply bundles of resources, wrapped
   up with a <a href="manifest.html"><code>manifest.json</code></a> file that
   describes the package's contents. The format of this file is generally stable,
   but occasionally breaking changes must be made to address specific issues.
   Developers should specify which version of the manifest specification their
   package targets by setting a <code>manifest_version</code> key in their
   manifests.
@@ skipping 19 matching lines @@
   either explicitly specify version <code>1</code>, or leave the key off
   entirely.
 </p>
 
 <p>
   At some point in the future, support for manifest version 1 will be removed.
   Ample warning will be provided, beginning with warnings when loading unpacked
   version 1 extensions, and escalating from there over time.
 </p>
 
+<p class="caution">
+  Setting <code>manifest_version</code> 2 in Chrome 17 or lower is not
+  recommended. If your extension needs to work in older versions of Chrome,
+  stick with version 1 for the moment. We'll give you ample warning before
+  version 1 stops working.
+</p>
+
 <h2>Changes between version 1 and 2</h2>
 
 <ul>
   <li>
     <p>
       A content security policy is set to <code>`script-src 'self'; object-src
       'self'</code> by default. This has a variety of impacts on developers,
       described at length in the <a href="contentSecurityPolicy.html">
       <code>content_security_policy</code></a> documentation.
     </p>
@@ skipping 86 matching lines @@
       </li>
       <li>
         <p>
           The <code>chrome.self</code> API has been removed. Use
           <a href="extension.html"><code>chrome.extension</code></a> instead.
         </p>
       </li>
     </ul>
   </li>
 </ul>