update clusterapi readme with node group limit info

this change is adding an extra note to the readme to help users
understand how the autoscaler works when it is outside the minimum and
mamximum limits. it is being added to help inform users and also because
the readme is embedded in clusterapi documentation.

Author: elmiko

cluster-autoscaler/cloudprovider/clusterapi/README.md

@@ -186,6 +186,13 @@ There are two annotations that control how a cluster resource should be scaled:
 The autoscaler will monitor any `MachineSet`, `MachineDeployment`, or `MachinePool` containing
 both of these annotations.
 
+> Note: The cluster autoscaler does not enforce the node group sizes. If a node group is
+> below the minimum number of nodes, or above the maximum number of nodes, the cluster
+> autoscaler will not scale that node group up or down. The cluster autoscaler can be configured
+> to enforce the minimum node group size by enabling the `--enforce-node-group-min-size` flag.
+> Please see [this entry in the Cluster Autoscaler FAQ](https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/FAQ.md#my-cluster-is-below-minimum--above-maximum-number-of-nodes-but-ca-did-not-fix-that-why)
+> for more information.
+
 > Note: `MachinePool` support in cluster-autoscaler requires a provider implementation
 > that supports the new "MachinePool Machines" feature. MachinePools in Cluster API are
 > considered an [experimental feature](https://cluster-api.sigs.k8s.io/tasks/experimental-features/experimental-features.html#active-experimental-features) and are not enabled by default.