Add a CONTRIBUTING file with a code of conduct

[brettcannon]

GitHub supports having a CONTRIBUTING.md where you can outline stuff like how to run the tests, etc. The main reason I would like to have one for this project is to have a code of conduct associated . My personal preference is for the PSF CoC, but any reasonable CoC would do.

If @daira and @takluyver can agree to a CoC then I can write a simple CONTRIBUTING.md that says this project falls under it.

[takluyver]

The PSF link was pointing to the Github help again - I've updated it to https://www.python.org/psf/codeofconduct/ - is that what you intended?

I'm fine with referring to the PSF CoC.

[brettcannon]

Yep, that's the CoC I was referring to.