Add examples to Rust cookbook

[nestordemeure]

The Rust cookbook has a section on statistics but, while other section points you to common crates, its examples are about computing the mean, median and std from scratch.

It might be a good place to advertise about ndarray-stats (and maybe a non ndarray based crate such as statistical).

[LukeMathWalker]

There is a PR open rust-lang-nursery/rust-cookbook#560

[AndyGauge]

The cookbook has traditionally been about displaying curated crates, originally selected through the libs team. I think this model is doing the community a disservice. If the ndarray team wants to showcase ndarray-stats as a better way of doing things, lets do it.