[prometheus addon] Add readme

cluster/addons/prometheus/README.md

+# Prometheus Add-on
+
+This add-on is an experimental configuration of k8s monitoring using Prometheus used for e2e tests.
+
+For production use check out more mature setups like [Prometheus Operator](https://github.com/coreos/prometheus-operator) and [kube-prometheus](https://github.com/coreos/prometheus-operator/tree/master/contrib/kube-prometheus).
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