[web-audio-api] PannerNode - include informative note on HRTF, point to reference/open examples (#54)

> Originally reported on W3C Bugzilla [ISSUE-21550](https://www.w3.org/Bugs/Public/show_bug.cgi?id=21550) Tue, 02 Apr 2013 16:25:41 GMT
> Reported by Olivier Thereaux
> Assigned to 

Per discussion at Audio WG f2f 2013-03-26

While HRTF is well documented in the audio processing world, we felt it would be valuable for our spec to e.g point to some open models, to ease implementation.

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https://github.com/WebAudio/web-audio-api/issues/54