Updated documentation to reference npm installation

This is such a minor thing but when I first went to the SuperTest documentation I didn't see an obviously way of installing. Naturally it was very easy to assume how to do so but sometimes modules available on GitHub do not always share the same name as a module within the npm repository.

Therefore, since I was already poking around, I added a quick note to the documentation. Feel free to re-word or even reject this specific pull request but I think this should at least be captured within the documentation.