Add docs for chrome.wallpaper API

chrome/common/extensions/docs/templates/intros/wallpaper.html

+<h2 id="manifest">Manifest</h2>
+<p>You must declare the "wallpaper" permission in the app
+<a href="manifest.html">manifest</a> to use the wallpaper API.
+For example:</p>
+<pre data-filename="manifest.json">
+{
+  "name": "My extension",
+  ...
+  <b>"permissions": [
+    "wallpaper"
+  ]</b>,
+  ...
+}</pre>
+
+<h2 id="overview-examples">Examples</h2>
+
+<p>
+For example, to set the wallpaper as the image at
+<code>http://example.com/a_file.png</code>, you can call
+<code>chrome.wallpaper.setWallpaper</code> this way:
+</p>
+
+<pre>
+chrome.wallpaper.setWallpaper(
+    {
+      'url': 'http://example.com/a_file.jpg',
+      'layout': 'CENTER_CROPPED',
+      'name': 'test_wallpaper'
+    }, function() {});
+</pre>