Add steps needed to run a basic performance test in benchmarks

It would be a nice addition to the devguide if the benchmarks section of runtests.rst contained the minimal commands needed to run pyperformance for comparing the potential performance diffs introduced by a change a user performed. Usually when a performance issue arises, the result of compare_to is posted for others to examine.

Specifically, if possible, a small section of the form "how to compare between commits" and with what options set would be ideal. Because pyperformance takes a considerable amount of time to finish, getting the right commands from the start would save people (myself included :-) quite some time.

Though it might be argued that this should go in the docs for performance I think having a small set of commands in the devguide with a see more pointing to the README.txt of the repo might be better.

Start with the benchmarks section of runtests.rst and review the pyperformance README.txt for the commands and options used with compare_to. Add a short “how to compare between commits” section with the minimal commands and a link to the performance documentation. Done means contributors can begin a comparison without needing to discover the command sequence themselves.