docs: add note about update script placeholder

index.html

@@ -2200,7 +2200,11 @@ https://...</pre>
         <p>
         You could also try to provide a <code>$(PKG)_UPDATE</code> section.
         However, that requires some experience and "feeling" for it.
-        So it is perfectly okay if you leave the <code>$(PKG)_UPDATE</code> section empty.
+        So it is perfectly okay if you leave a placeholder:
+        <pre>define $(PKG)_UPDATE
+    echo 'TODO: write update script for $(PKG).' >&2;
+    echo $($(PKG)_VERSION)
+endef</pre>
         We'll fill that in for you.
         It's a funny exercise.
         </p>