[web-audio-api] Introduction: link "use cases" to the stable Use Cases document (#32)

> Originally reported on W3C Bugzilla [ISSUE-21512](https://www.w3.org/Bugs/Public/show_bug.cgi?id=21512) Tue, 02 Apr 2013 12:21:56 GMT
> Reported by Olivier Thereaux
> Assigned to 

Per discussion at Audio WG f2f 2013-03-26:

The Introduction section mentions the use cases and requirements document. The link should be pointing to http://www.w3.org/TR/webaudio-usecases/ and that spec/Note added to the list of references.

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https://github.com/WebAudio/web-audio-api/issues/32