* Provide example to utilize k8s_external plugin

The example provides a specific use case of k8s_external and may help others to grasp `k8s_external`'s capabilities.

* Update plugin/k8s_external/README.md
Co-Authored-By: Chris O'Haver <cohaver@infoblox.com>

* Update plugin/k8s_external/README.md
Co-Authored-By: Chris O'Haver <cohaver@infoblox.com>