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.

從 runtests.rst 的 benchmarks 部分開始，並查看 pyperformance README.txt，了解與 compare_to 搭配使用的命令和選項。新增一個簡短的「如何比較不同 commit」部分，其中包含最少的命令以及指向效能文件的連結。當貢獻者無需自行摸索命令順序即可開始比較時，即視為完成。