Browser Compatibility in documentation

[jasonkarns]

I know that part of this has been discussed ad nauseum in #104. However, I think it should be made clear in the readme whether or not html5shiv is safe to include in modern browsers.

As long as the readme recommends referencing the script via conditional comment (not a bad thing), the question of whether the script itself is safe is up in the air.

A simple blurb that states what happens in modern browsers would clear this up.