Add documentation explaining the difference between CRDs and API aggregation layer

Author: pegasas

content/en/docs/concepts/extend-kubernetes/api-extension/_index.md

@@ -2,3 +2,10 @@
 title: Extending the Kubernetes API
 weight: 30
 ---
+
+Custom resources are extensions of the Kubernetes API. Kubernetes provides two ways to add custom resources to your cluster:
+
+- The [CustomResourceDefinition](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/)
+API resource allows you to define a CRD object creates a new custom resource with a name and schema that you specify.
+The Kubernetes API serves and handles the storage of your custom resource. CRDs allow users to create new types of resources without adding another API server. 
+- The [aggregation layer](/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/) sits behind the primary API server, which acts as a proxy. This arrangement is called API Aggregation(AA). To users, the Kubernetes API appears extended.