Document notebook format

Our project is auto-generating notebook json to be imported to Zeppelin (https://github.com/aishfenton/Vegas).

It would be good to have the notebook json schema documented and versioned, so that there's some guarantee that if the schema changes we'll know about it, and be able to update our end.

Here's an example of what Jupyter does:
https://nbformat.readthedocs.io/en/latest/