doc: add more info to benchmark/README.md
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 <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Evan Lucas <ad0310dc5270cefb6ee6879263270aed9b1e9982@me.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node