Add instructions to enable API group to VolumeAttributesClass

Author: AndrewSirenko

content/en/docs/concepts/storage/volume-attributes-classes.md

@@ -29,6 +29,12 @@ If you want to test the feature whilst it's beta, you need to enable the `Volume
 --feature-gates="...,VolumeAttributesClass=true"
 ```
 
+You will also have to enable the `storage.k8s.io/v1beta1` API group through the `kube-apiserver` [runtime-config](https://kubernetes.io/docs/tasks/administer-cluster/enable-disable-api/). You use the following command line argument: 
+
+```
+--runtime-config=storage.k8s.io/v1beta1=true
+```
+
 You can also only use VolumeAttributesClasses with storage backed by
 {{< glossary_tooltip text="Container Storage Interface" term_id="csi" >}}, and only where the
 relevant CSI driver implements the `ModifyVolume` API.