document how to use `KubernetesExecutor`

[thesuperzapper]

Currently we natively support the KubernetesExecutor and CeleryKubernetesExecutor executors, but there is nothing to reflect this in the main README.md.

We should add a new section called something like: How to use KubernetesExecutor?, and outline the values which are specific to that mode.

[thesuperzapper]

We could also setup a minikube_kubernetes_executor example in the examples folder:
https://github.com/airflow-helm/charts/tree/main/charts/airflow/examples

Note, don't forget to update the examples section of the main README.