DOC/DEV: document approach to bisect regressions
@simonjayhawkins you have been doing great work on finding the causes of bugs/regressions! 
To make it easier for other contributors to also do this, I think it would be nice to document this is the contributing docs (even though it is relying on git, using it with pandas (which needs to be rebuilt for each commit) requires a bit of custom setup I think?)