Add documentation regarding how Criterion does its measurements

This is really important for people writing papers about Haskell who use Criterion to perform measurements. I'm helping a team that is using a modified version of Criterion to measure the energy consuption of benchmarks (we plan to file a pull request when it's done).

It's mainly to explain the statistical reasoning behind the [`runBenchmark` function from `Criterion.Measurement`](https://github.com/bos/criterion/blob/f5580029863744f4a58b333198e708ac32e43e17/Criterion/Measurement.hs#L95).