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.

从 runtests.rst 的 benchmarks 部分开始，并查看 pyperformance README.txt，了解与 compare_to 一起使用的命令和选项。添加一个简短的“如何比较不同 commit”部分，其中包含最少的命令以及指向性能文档的链接。当贡献者无需自行摸索命令顺序即可开始比较时，即视为完成。