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.

[PFWhite]

This was super useful to me. If you are looking how to set up the KubernetesExecutor using the apache-airflow/airflow helm chart then read these commits to get it right.

[thesuperzapper]

We should also explain that your KubernetesExecutor pod_template is dependent on Secret/airflow-config-envs, so you must run all your task Pods inside the namespace you installed airflow into.

So user's cant override AIRFLOW__KUBERNETES__NAMESPACE, or any other way which you can have airflow submit KubernetesExecutor Pods into a non-default namespace.