Added instructions on configuring adapter to collect ingress metrics (#34)

Signed-off-by: Arjun Naik <arjun.rn@gmail.com>

how-to/skipper_setup.md

@@ -0,0 +1,40 @@
+# Skipper Prometheus Metrics Collection
+
+The skipper-ingress pods should be configured to be scraped by Prometheus. This
+can be done by Prometheus service discovery using discovery of Kubernetes services
+or Kubernetes pods:
+
+```yaml
+apiVersion: v1
+kind: Service
+metadata:
+  annotations:
+    prometheus.io/path: /metrics
+    prometheus.io/port: "9911"
+    prometheus.io/scrape: "true"
+  labels:
+    application: skipper-ingress
+  name: skipper-ingress
+spec:
+  ports:
+  - port: 80
+    protocol: TCP
+    targetPort: 9999
+  selector:
+    application: skipper-ingress
+  type: ClusterIP
+```
+This [configuration](https://github.com/zalando-incubator/kubernetes-on-aws/blob/dev/cluster/manifests/prometheus/configmap.yaml#L69)
+shows how prometheus is configured in our clusters to scrape service endpoints.
+The annotations `prometheus.io/path`, `prometheus.io/port` and `prometheus.io/scrape`
+instruct Prometheus to scrape all pods of this service on the port _9911_ and
+the path `/metrics`.
+
+When the `kube-metrics-adapter` is started the flag `--prometheus-server` should be set so that
+the adapter can query prometheus to get aggregated metrics. When running in kubernetes it can
+be the service address of the prometheus service like `http://prometheus.kube-system`.
+
+With these settings the `kube-metrics-adapter` can provide `request-per-second` metrics for ingress
+objects which are present in the cluster. The prometheus instances scrape the metrics from
+the `skipper-ingress` pods. The adapter then queries prometheus to get the metric and then
+provides them to the API server when requested.