Document how to update and deploy notebooks

[bsipocz]

The astroml notebooks are rendered on the website which is hosted in a separate repo. Document how the process works.

[bsipocz]

The bare minimum documentation is already present in the repo README, but we should also link to more detailed how-to, e.g. https://github.com/numpy/numpy-tutorials#contributing and their linked resources would be a good place to start.

We should also test it whether this amount of information is enough to enable people to add their new content into the repo.