Dask-xgboost example for dask-examples

It would be nice to see an example using the Dask/XGBoost handoff for parallel training and predicting. This is a common question and so would likely have high value.

It would also be useful for this to be smoothly runnable on [dask-examples](https://github.com/dask/dask-examples#contributing). Presumably we'll have to use a few processes within a LocalCluster and be careful not to blow out RAM on the small containers (XGBoost can be a bit greedy).