Include benchmarks in README.md

[philippeitis]

It would be helpful to include sample benchmark results (probably as X times speedup at each size, maybe in graph format?) in the README so that users could have context for what performance gains can be expected (and whether everything performs as expected).

If I have time, I can compile some results, but I don't know if there's a convenient criterion API to generate such a table automatically (it does produce JSON files, which could be tablulated using a script).