examples/audio/04-multiple-streams: Note about web browser audio in README.

examples/audio/04-multiple-streams/README.txt

@@ -1,3 +1,5 @@
+If you're running this in a web browser, you need to click the window before you'll hear anything!
+
 This example code loads two .wav files, puts them an audio streams and binds
 them for playback, repeating both sounds on loop. This shows several streams
 mixing into a single playback device.