Adding cautionary note about manifest_version 2 and Chrome 17.

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

 <div id="pageData-name" class="pageData">Formats: Manifest Files</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 Every extension, installable web app, and theme has a
 <a href="http://www.json.org">JSON</a>-formatted manifest file,
 named <code>manifest.json</code>,
 that provides important information.
 </p>
 
@@ skipping 682 matching lines @@
 version in Chrome 18 can either explicitly specify version <code>1</code>, or
 leave the key off entirely.
 </p>
 
 <p>
 The changes between version 1 and version 2 of the manifest file format are
 described in detail in <a href="manifestVersion.html">the
 <code>manifest_version</code> documentation.</a>
 </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>
+
 <h3 id="web_accessible_resources">web_accessible_resources</h3>
 
 <p>
 An array of strings specifying the paths (relative to the package root) of
 packaged resources that are expected to be usable in the context of a web page.
 For example, an extension that injects a content script with the intention of
 building up some custom interface for <code>example.com</code> would whitelist
 any resources that interface requires (images, icons, stylesheets, scripts,
 etc.) as follows:
 </p>
@@ skipping 31 matching lines @@
 </a> 2 or above are <strong>blocked by default</strong>, and must be whitelisted
 for use via this property.
 </p>
 
 <p>
 Resources inside of packages using <code>manifest_version</code> 1 are available
 by default, but <em>if</em> you do set this property, then it will be treated as
 a complete list of all whitelisted resources. Resources not listed will be
 blocked.
 </p>