Document current lack of support for Wav Extensible

[weefuzzy]

Some programmes (looking at you Sox and Steinberg) make happy use of the Wave Extensible header chunk for stuff that isn't 16-bit fixed or 32-bit float mono/stereo. This is apparently formally correct as well. IAC, HISSTools AudioFile doesn't support reading this chunk, so we should say so up front (and see about perhaps changing this with a PR).

[weefuzzy]

see #5 for related issue of this being not properly reported at runtime

[AlexHarker]

This may have changed/be changing with header-only inclusion - something else to test once that work is validated.