README: explain the screen-share indicator for audio

Gearslip captures playback, not the screen, but Android's consent
dialog and persistent indicator look identical either way. Note why
the indicator appears, and correct the adjacent no-adb claim to
mention the two optional audio commands as the sole exception.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -23,6 +23,11 @@ than Google's own can talk to a car head unit.
   tested against several real navigation and media apps.
 - **Media playback works.** Gearslip lets you browse and control car-enabled media apps
   from the car screen, with lyrics and playback state, over the same host connection.
+- **Audio reaches the car by capturing playback, not by mirroring the screen.** Android
+  requires a screen-share style consent dialog and indicator for this, even though
+  Gearslip never reads the screen; that grant is the only way a non-privileged app can
+  tap another app's audio. The consent normally repeats every drive; the app's own setup
+  steps include two optional adb commands that reduce it to a one-time grant.
 - **A real head unit needs a certificate that Google controls.** A head unit accepts only
   a phone identity that chains to Google's Automotive Link certificate authority, and
   Google alone issues those certificates. Gearslip does not ship one. Without it,
@@ -30,7 +35,9 @@ than Google's own can talk to a car head unit.
   connection. A leaked certificate exists that older head units accept; newer firmware
   and Google's reference tooling reject it.
 - **Gearslip needs no adb, root, or Shizuku.** Every feature relies on an ordinary
-  Android permission or an on-device consent dialog.
+  Android permission or an on-device consent dialog. The two adb commands mentioned
+  above, for audio, are the sole exception: both are optional, and both only remove a
+  dialog that the driver would otherwise see once per drive.
 
 ## Building