Add docstring requirements to contribution guidelines

[meatballs]

One comment at the PyCon UK sprint was that it's not clear from our guidelines that we expect numpy style docstrings for methods and functions.

We do have #347 to clean up our existing codebase, but we've not been explicit in our expectations for new code.

[drvinceknight]

👍 (Just an FYI for whoever works on this, the numpy guideline does not extend to strategies for which numpy style ---- etc under section headings breaks the autodoc as it conflicts with the rst syntax).