Include benchmarks in README

[chriskuehl]

It'd be nice to have a small benchmark (graph or something) comparing dumb-pypi to maybe pypiserver and pypicloud with a few different numbers of files.

Obviously the dumb-pypi benchmark is "sort of" cheating because it's just serving a static file and will take approximately 0 milliseconds, but I think people will be surprised to see pypicloud taking ~700ms for only 200 files (not that many when you have multiple formats or wheels per version!).

And wouldn't it be nice if your prod PyPI took approximately 0 milliseconds to respond? :P