add documentation for the example dashboard (#84)

README.md

@@ -131,6 +131,22 @@ elasticsearch_exporter --help
 
 We provide examples for [Prometheus](http://prometheus.io) [alerts and recording rules](examples/prometheus/elasticsearch.rules) as well as an [Grafana](http://www.grafana.org) [Dashboard](examples/grafana/dashboard.json) and a [Kubernetes](http://kubernetes.io) [Deployment](examples/kubernetes/deployment.yml).
 
+The example dashboard needs the [node_exporter](https://github.com/prometheus/node_exporter) installed. In order to select the nodes that belong to the elastsearch cluster, we rely on a label `cluster`.
+Depending on your setup, it can derived from the plattform metadata:
+
+For example on [GCE](https://cloud.google.com)
+
+```
+- source_labels: [__meta_gce_metadata_Cluster]
+  separator: ;
+  regex: (.*)
+  target_label: cluster
+  replacement: ${1}
+  action: replace
+```
+
+Please refer to the [Prometheus SD documentation](https://prometheus.io/docs/operating/configuration/) to see which metadata labels can be used to create the `cluster` label.
+
 ## Credit & License
 
 `elasticsearch_exporter` is maintained by the nice folks from [JustWatch](https://www.justwatch.com/)