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.

runtests.rst の benchmarks セクションから始め、compare_to で使用するコマンドとオプションについて pyperformance の README.txt を確認してください。最小限のコマンドとパフォーマンスドキュメントへのリンクを含む、短い「コミット間の比較方法」セクションを追加してください。コントリビューターがコマンドの順序を自分で調べなくても比較を開始できれば完了です。