fix music navs

README.md

@@ -8,7 +8,8 @@ Everything it does fits on one screen:
 
 - **Volume** up/down for the Home 400 and the Living Room pair. A tap lands
   on the next multiple of `VOLUME_STEP` — from 23 it goes to 25, not 28 —
-  so the levels stay round. Hold to keep moving.
+  so the levels stay round. Hold to keep moving, or drag the level along
+  its bar to set it outright.
 - **Play or pause** either room, or skip a track, while it is playing
   Spotify — the buttons only show up then, since an AVR input has nothing
   to pause or skip. A room that is grouped shares the AVR's
@@ -134,10 +135,15 @@ key is missing (they usually match). `GET /api/spotify/devices?account=fifou`
 lists what Spotify actually calls each device, if a room's button ever says
 one isn't visible.
 
-A room's play/pause and next buttons only appear while HEOS reports it is
-playing (or paused on) Spotify. To see what HEOS reports for a room, use
+A room's previous, play/pause and next buttons only appear while HEOS
+reports it is playing (or paused on) Spotify, and one of your `SPOTIFY_ACCOUNTS` is the
+one playing it — the same match that borders its button. To see what HEOS reports for a room, use
 `GET /raw/player/get_now_playing_media?pid=<pid>` with a pid from
-`/api/targets`: Spotify shows up as `"sid": 4`.
+`/api/targets`: Spotify shows up as `"sid": 4`. The song, artist and cover
+under each room's name come from that same reply (`song`, `artist`,
+`image_url`), whichever of them HEOS fills in. They stay while paused and go
+once the room stops. An AVR input shows none, since its "song" is just the
+input's name.
 
 ## Add it to the iOS home screen