explain over-the-air delta updates

static/build.html

@@ -699,6 +699,11 @@ cd ../..</pre>
         ├── crosshatch-target_files-2019.10.07.21.zip
         └── crosshatch-testing</pre>
 
+            <p>Incremental updates are uploaded alongside the update packages and update metadata
+            on the static web server used as an update server. The update client will
+            automatically check for an incremental update and use it if available. No additional
+            metadata is needed to make incremental updates work.</p>
+
             <h2 id="prebuilt-code">
                 <a href="#prebuilt-code">Prebuilt code</a>
             </h2>