mobile: add a link to the audio package

The audio package runs on Android now, we can list it on README.

Change-Id: I2ff03f025328a1d0e6ffc12b860ace9c980dc422
Reviewed-on: https://go-review.googlesource.com/7521
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

README.md

@@ -50,6 +50,7 @@ documentation serves as a starting point:
 - [mobile/app](http://godoc.org/golang.org/x/mobile/app)
 - [mobile/gl](http://godoc.org/golang.org/x/mobile/gl)
 - [mobile/sprite](http://godoc.org/golang.org/x/mobile/sprite)
+- [mobile/audio](http://godoc.org/golang.org/x/mobile/audio)
 - [mobile/cmd/gobind](http://godoc.org/golang.org/x/mobile/cmd/gobind)
 
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