doc: add more info to benchmark/README.md

Adds info on the `wrk` prerequisite for http benchmarks and how to
run benchmarks with options.

PR-URL: https://github.com/iojs/io.js/pull/629


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>