More detailed usage example in README.md

The current code snippet in README.md is rather confusing and doesn't reflect relevant implementation details, such as how to instantiate the BasicPitch class, or clarifying that the AudioContext's sampling rate must be 22050. The provided example (which is admittedly lengthly) is copy-paste ready, and includes a docstring + explanatory comments.

This was really useful!

One more detail is that the file must be mono.