web README: installing the parents' app

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/README.md

@@ -154,8 +154,13 @@ service worker in `public/sw.js`. On iOS, Share → *Zum Home-Bildschirm*; it th
 full-screen with no Safari chrome, because iOS reads the `apple-mobile-web-app-*` meta
 tags in `index.html` rather than the manifest.
 
-Two things worth knowing:
+A few things worth knowing:
 
+- **Install from `?parentMode=1` to get a parents' app.** iOS starts an installed app at
+  the manifest's `start_url`, not at the address it was added from, so the plain
+  manifest's `/` would drop the flag. Opened with it, `index.html` swaps in
+  `public/manifest-parents.webmanifest` (`start_url: /?parentMode=1`, named "Musik
+  Eltern"). An app installed before this change has to be removed and added again.
 - **The service worker only registers over HTTPS or on `localhost`.** At
   `http://<mouse>:8080` on the LAN, iOS will still install the icon and run it
   standalone - that part is the meta tags - but there is no offline caching. Put the