add readme file for cluster-api provider

This change adds a readme which describes the basic operation of the
cluster-api provider.

Author: elmiko

cluster-autoscaler/cloudprovider/clusterapi/README.md

@@ -0,0 +1,54 @@
+# Cluster Autoscaler on Cluster API
+
+The cluster autoscaler on [Cluster API](https://cluster-api.sigs.k8s.io/) uses
+the [cluster-api project](https://github.com/kubernetes-sigs/cluster-api) to
+manage the provisioning and de-provisioning of nodes within a Kubernetes
+cluster.
+
+## Kubernetes Version
+
+The cluster-api provider requires Kubernetes v1.16 or greater to run the
+v1alpha3 version of the API.
+
+## Starting the Autoscaler
+
+To enable the Cluster API provider, you must first specify it in the command
+line arguments to the cluster autoscaler binary. For example:
+
+```
+cluster-autoscaler --cloud-provider=clusterapi
+```
+
+Please note, this example only shows the cloud provider options, you will
+most likely need other command line flags. For more information you can invoke
+`cluster-autoscaler --help` to see a full list of options.
+
+## Enabling Autoscaling
+
+To enable the automatic scaling of components in your cluster-api managed
+cloud there are a few annotations you need to provide. These annotations
+must be applied to either [MachineSet](https://cluster-api.sigs.k8s.io/developer/architecture/controllers/machine-set.html)
+or [MachineDeployment](https://cluster-api.sigs.k8s.io/developer/architecture/controllers/machine-deployment.html)
+resources depending on the type of cluster-api mechanism that you are using.
+
+There are two annotations that control how a cluster resource should be scaled:
+
+* `cluster.k8s.io/cluster-api-autoscaler-node-group-min-size` - This specifies
+  the minimum number of nodes for the associated resource group. The autoscaler
+  will not scale the group below this number. Please note that currently the
+  cluster-api provider will not scale down to zero nodes.
+
+* `cluster.k8s.io/cluster-api-autoscaler-node-group-max-size` - This specifies
+  the maximum number of nodes for the associated resource group. The autoscaler
+  will not scale the group above this number.
+
+The autoscaler will monitor any `MachineSet` or `MachineDeployment` containing
+both of these annotations.
+
+## Specifying a Custom Resource Group
+
+By default all Kubernetes resources consumed by the Cluster API provider will
+use the group `cluster.x-k8s.io`, with a dynamically acquired version. In
+some situations, such as testing or prototyping, you may wish to change this
+group variable. For these situations you may use the environment variable
+`CAPI_GROUP` to change the group that the provider will use.