Updated hpa docs.

docs/user-guide/horizontal-pod-autoscaler.md

@@ -75,6 +75,9 @@ Please note that if some of the pod's containers do not have CPU request set,
 CPU utilization for the pod will not be defined and the autoscaler will not take any action.
 Further details of the autoscaling algorithm are given [here](../design/horizontal-pod-autoscaler.md#autoscaling-algorithm).
 
+Autoscaler uses heapster to collect CPU utilization.
+Therefore, it is required to deploy heapster monitoring in your cluster for autoscaling to work.
+
 Autoscaler accesses corresponding replication controller or deployment by scale sub-resource.
 Scale is an interface which allows to dynamically set the number of replicas and to learn the current state of them.
 More details on scale sub-resource can be found [here](../design/horizontal-pod-autoscaler.md#scale-subresource).

docs/user-guide/horizontal-pod-autoscaling/README.md

@@ -42,6 +42,11 @@ In this document we explain how this feature works by walking you through an exa
 ## Prerequisites
 
 This example requires a running Kubernetes cluster and kubectl in the version at least 1.1.
+[Heapster](https://github.com/kubernetes/heapster) monitoring needs to be deployed in the cluster
+as horizontal pod autoscaler uses it to collect metrics
+(if you followed [getting started on GCE guide](../../../docs/getting-started-guides/gce.md),
+heapster monitoring will be turned-on by default).
+
 
 ## Step One: Run & expose php-apache server
 
@@ -214,4 +219,4 @@ As we see, in the presented case CPU utilization dropped to 0, and the number of
 
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