Add entropy.md to discuss fingerprinting, and compat concerns

[dtig]

@penzn Good question, the ones I have here are mostly off the cuff estimates of which instructions could possible cause compat issues when the same Wasm binary is run on different hardware/browsers from taking into account the instructions proposed, usage patterns and the semantics of the instructions. I'm curious for thoughts from @justinmichaud and @eqrion for thoughts about quantifying risk of compat issues.

[justinmichaud]

This is a very helpful document, thank you!