Add MIT LICENSE.md

@Philzen @hyperlink

Fixes #5

The project is marked as MIT license here:
https://github.com/Philzen/measurement.js/blob/587018eb512dcf320beeb0b18c795f180e7c38a1/package.json#L11

This adds the `LICENSE.md` file and updates the `README.md` to include a reference to it.