Add light documentation structure and README for kubernetes

docs/kubernetes/README.md

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+# Installing Mailu with Kubernetes #
+
+# Prerequisites #
+
+The resource configurations in this folder assume that you have [Kubernetes Ingress](https://kubernetes.io/docs/concepts/services-networking/ingress/) set up for your cluster. If you are not using the [NGINX Ingress Controller for Kubernetes](https://github.com/kubernetes/ingress/tree/master/controllers/nginx), please ensure that the configuration specified in the file matches your set up.
+
+# How to use #
+
+Using the resource configurations is simple:
+
+1. `kubectl apply -f kubernetes-nginx-ingress-controller.yaml` to configure an ingress controller with the proper settings. (If you have one set up already you may need to port the configuration to your own ingress).
+2. `kubectl apply -f kubernetes-mailu.yaml` to create the resources required to run Mailu.
+
+Based on the configuration, your Mailu instance should be available at `mail.<hostname>.tld/admin` (note that visiting just `mail.<hostname>.tld` will likely result in a 404.