explain other repos are referenced by the manifest

static/source.html

@@ -28,6 +28,10 @@
                 <li><a href="https://github.com/GrapheneOS/branding_tools">Scripting for managing branding</a></li>
                 <li><a href="https://github.com/GrapheneOS/platform_manifest">Manifest for OS repositories</a></li>
             </ul>
+            <p>The operating system includes hundreds of repositories. The manifest repository
+            references all of these. Most of these are used unmodified from the Android Open
+            Source Project (AOSP). A few dozen of the repositories are either forked from AOSP or
+            are unique to GrapheneOS.</p>
         </div>
     </body>
 </html>