Add autopilot customer churn notebook (#1092)

* Add autopilot customer churn notebook

Similar to the XGBoost customer churn notebook, but using autopilot
instead.

* Fix variable rename and format string

* Limit candidates to 20 and improve text

So that the automl job completes within a few minutes we
now limit the max candidates to 20. We've also added a note
regarding the nature of the task, BinaryClassification, and
how AutoPilot is able to auto detect it.

Author: juliodelgadoaws