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.

Commencez par la section benchmarks de runtests.rst et consultez le README.txt de pyperformance pour connaître les commandes et les options utilisées avec compare_to. Ajoutez une courte section « comment comparer entre des commits » avec les commandes minimales et un lien vers la documentation sur les performances. C’est terminé lorsque les contributeurs peuvent commencer une comparaison sans devoir trouver eux-mêmes la séquence de commandes.