Add documentation about running built benchmarks

[ehpor]

@ivalaginja The benchmarks are compiled alongside the Python library when you run python setup.py develop. You can run them from the build/bin folder.

@ehpor Should this be contained in the documentation somewhere or is this common knowledge? I suspect it's the latter, might just not be true for an audience like me 😬

Originally posted by @ivalaginja in #236 (comment)