docs: mention passthrough

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

README.md

@@ -48,7 +48,7 @@ The page itself gets no extra controls, so video lightboxes and overlays keep wo
 
 **In VR**
 
-A controls panel floats below the video: play/pause, the seek bar, skip 10 seconds back or forward, mute and volume, the projection (‹ › to switch), Recenter and Exit VR.
+A controls panel floats below the video: play/pause, the seek bar, skip 10 seconds back or forward, mute and volume, the projection (‹ › to switch), passthrough, Recenter and Exit VR.
 
 | To | Do this |
 |---|---|
@@ -59,6 +59,7 @@ A controls panel floats below the video: play/pause, the seek bar, skip 10 secon
 | Resize the flat screen | Drag its bottom right corner, or push the thumbstick left or right while holding it |
 | Play or pause | Click the flat screen |
 | Skip 10 seconds back or forward | Double click the left or right third of the flat screen, like on YouTube's mobile app; each further click skips another 10 seconds |
+| See the room around the screen | Press the passthrough button (the headset icon). It works where the browser supports AR with passthrough, e.g. Chrome Canary on the Steam Frame ([install guide](https://github.com/phit/chromium-webxr-steam-frame/tree/google#install-on-the-frame)), and is greyed out elsewhere |
 | Put everything back in front of you | Recenter on the panel. On 360° and 180° videos it also turns the front of the video to where you look |
 
 ### Videos to try