Example Explaining Serialization

Lots of good discussion here: https://github.com/jupyter-widgets/ipywidgets/issues/2922


Points to hit:
1. How to serialize custom widgets
2. How add custom serializers
3. How to serialize things like numpy arrays (likely use https://github.com/vidartf/ipydatawidgets)
4. Talk about the traittypes library: https://github.com/jupyter-widgets/traittypes