benchmark: add info about required Unix tools

This adds note to README.md about Unix tools being
required by some benchmarks

PR-URL: https://github.com/nodejs/node/pull/8788


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>