Document the `rayon` feature

[brson]

This crate can optionally enable rayon. Unfortunately we don't have guidelines on the proper way to do this.

I'd suggest adding a "Parallelism" section to crate docs, and then writing a short paragraph explaining how to turn on the feature and what it does. Depending on the outcome of https://github.com/alexcrichton/gcc-rs/issues/195 there might end up being other things to say too.