Add docs for site.static_files

site/docs/variables.md

@@ -112,6 +112,16 @@ following is a reference of the available data.
 
       </p></td>
     </tr>
+    <tr>
+      <td><p><code>site.static_files</code></p></td>
+      <td><p>
+
+        A list of all static files (i.e. files not processed by Jekyll's
+        converters or the Liquid renderer). Each file has two properties:
+        <code>path</code> and <code>modified_time</code>
+
+      </p></td>
+    </tr>
     <tr>
       <td><p><code>site.categories.CATEGORY</code></p></td>
       <td><p>