Doc: Missing js-xtract

[X-Raym]

Hi,

First thanks for these great resources, your Spectral Centroid waveform colorization is very nice ! I asked for something like that in WaveSurfer.js years ago, so maybe your code could help realize this feature!

This library just miss some global documentation, like the way it needs js-xtract to works.

Well, I think it is a great resource, and it would deserve a bit more doc: screenshots, keywords etc... Other things that the doc research it is linked too http://annaxambo.me/pub/Roma_et_al_2018_A_Javascript_library.pdf (as good as it is !)

Cheers !