[Docs] How to grab the music.

[carlsmith]

The docs explain how to modulate the audio really well, but it's not clear what sources we can use or how to access them.

I'm figuring things out from the demo, but it would be helpful to have a section in the docs explaining how to access the mic and buffers.

[adamfr33man]

Might be worth writing up what you've found out when your done @carlsmith, I'm sure the author would appreciate a pull request :)

[dashersw]

Hello! In the example, we use the StreamInput class, which takes in whatever line-in method there is available in the browser. It basically uses navigator.getUserMedia. There's also a FileInput class which lets you use a sound file as a source, so you can play back recordings. The input/output classes are available in the io folder.