Add cas docs

docs/cluster_spec.md

@@ -616,6 +616,20 @@ spec:
 
 For more details on `horizontalPodAutoscaler` flags see the [official HPA docs](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/) and the [Kops guides on how to set it up](horizontal_pod_autoscaling.md).
 
+## Cluster autoscaler
+
+{{ kops_feature_table(kops_added_default='1.19', k8s_min='1.15') }}
+
+Cluster autoscaler can be enabled to automatically adjust the size of the kubernetes cluster.
+
+```yaml
+spec:
+  cluster:
+    clusterAutoscaler: {}
+```
+
+Read more about cluster autoscaler in the [official documentation](https://github.com/kubernetes/autoscaler/tree/master/cluster-autoscaler).
+
 ###  Feature Gates
 
 Feature gates can be configured on the kubelet.