Document strictMode for not including <title> as h1

[paulrohrbeck]

It would be nice to get the strictMode property added to the README.

It threw me off for a minute that it was showing an error for having two h1s when in reality it was just the <title> being treated as an <h1>

PS: Thanks so much for this library, super helpful!

[Bishwas-py]

I actually forgot my own code, haha. I was like, what the heck is strictMode? And then boom, realization. Got it, man, will do. You're awesomeness is flowing all over github. Perfect contributor!