Add note re: modern browser compatibility to README

As discussed in #60, I specifically call out the `bind` function. It seems bad form to shim all of ES5 in legacy environments *just for testing*.

I realize that there is [an issue with IE < 9 and asserts in Chai](https://github.com/chaijs/chai/issues/117), so feel free to accept or not if you feel this will lead to false hopes of compatibility.