If you are developing specialized macros for your own set of problems you could also put them in webwork-open-problem-library/OpenProblemLibrary/macros. I don't see any additional statistics type macros there at this time.
We're still working on the docs for "how to become a developer" but some preliminaries are at:
http://webwork.maa.org/wiki/Github#Contributing_to_WeBWorK
Basically you should sign up for your own github.com account (they are free) and when something is ready for inspection/distribution send a pull request to openwebwork.
For development start using the webwork2-dev and pg-dev repositories on openwebwork. They are fairly stable and represent the stable tip of development. The webwork and pg repositories we are using for distribution
for those who don't plan to develop code but want a more seasoned release.
Have fun and let us know when you've got something.
Take care,
Mike