Add blackbox-exporter to included components list (#2412)

This is clearly available within `manifests/`, but isn't listed in the README. Including this now for clear communication of what can be expected using this package.

README.md

@@ -16,6 +16,7 @@ Components included in this package:
 * Highly available [Prometheus](https://prometheus.io/)
 * Highly available [Alertmanager](https://github.com/prometheus/alertmanager)
 * [Prometheus node-exporter](https://github.com/prometheus/node_exporter)
+* [Prometheus blackbox-exporter](https://github.com/prometheus/blackbox_exporter)
 * [Prometheus Adapter for Kubernetes Metrics APIs](https://github.com/kubernetes-sigs/prometheus-adapter)
 * [kube-state-metrics](https://github.com/kubernetes/kube-state-metrics)
 * [Grafana](https://grafana.com/)