Add cert-manager and let's encrypt to the k8s-cluster

This change contains a new module which will be applied after the
kubernetes-cluster has been created. It will install the cert-manager in
it and add let's encrypt as a ClusterIssuer to the cluster. That setup
allows me to simply issue certificates for all services in the cluster.

k8s/README.adoc

@@ -0,0 +1,21 @@
+= k8s
+:icons: font
+
+This project is responsible for providing general services in the kubernetes-cluster.
+
+== Setup
+
+// tag::setup[]
+[WARNING]
+Make sure `config.auto.tfvars` with all the needed configuration-secrets is present otherwise the module cannot be applied!
+The file is savely stored in the password-manager.
+
+[source,bash]
+----
+terraform init # <1>
+terraform apply # <2>
+----
+
+<1> Init the terraform modules if necessary
+<2> Create services in the cluster
+// end::setup[]