Trivial improvements to hphp/test/frameworks/perf

Summary: - fix markdown formatting in 'Usage' section of README.md
- make Usage section say bin/php-cgi, not bin/php
- make Usage section say it will run as a regular user
- specify a log file (/dev/null) to siege in warmup mode
- add a timestmap to print_progress

refs facebook/hhvm#3664

Reviewed By: @JoelMarcey

Differential Revision: D1536535