Add information re: .jekyll-metadata

site/_docs/structure.md

@@ -32,6 +32,7 @@ A basic Jekyll site usually looks something like this:
 ├── _data
 |   └── members.yml
 ├── _site
+├── .jekyll-metadata
 └── index.html
 {% endhighlight %}
 
@@ -155,6 +156,22 @@ An overview of what each of these does:
         </p>
       </td>
     </tr>
+    <tr>
+      <td>
+        <p><code>.jekyll-metadata</code></p>
+      </td>
+      <td>
+        <p>
+
+          This is file helps Jekyll keep track of which files have not been
+          modified since the site was last built, and which files will need to
+          be regenerated on the next build. This file will not be included in
+          the generated site. It’s probably a good idea to add this to your
+          <code>.gitignore</code> file.
+
+        </p>
+      </td>
+    </tr>
     <tr>
       <td>
         <p><code>index.html</code> and other HTML, Markdown, Textile files</p>