Document how benchmark.js works

[jmarcosfer]

I have been working with this library a few weeks now and while I have managed to use it with the simple API docs provided here, the only seemingly official source I have been able to find on how it works is this article by its authors.

If you're trying to decide with benchmarking library to use, or why benchmark.js is worth using, I think it's necessary to know how it measures performance and why. Why is that not explained as part of the official documentation?