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.

Comienza con la sección benchmarks de runtests.rst y revisa el README.txt de pyperformance para consultar los comandos y las opciones que se usan con compare_to. Añade una sección breve «cómo comparar entre commits» con los comandos mínimos y un enlace a la documentación de performance. Se considera completado cuando los colaboradores pueden iniciar una comparación sin tener que descubrir por sí mismos la secuencia de comandos.