add some description of aggregation layer

Author: pegasas

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

@@ -10,4 +10,7 @@ Custom resources are extensions of the Kubernetes API. Kubernetes provides two w
   schema that you specify.
   The Kubernetes control plane serves and handles the storage of your custom resource. CRDs allow you to
   create new types of resources for your cluster without writing and running a custom 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.
+- 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), which allows you to provide specialized implementations for your custom resources by writing and
+deploying your own API server.
+The main API server delegates requests to your API server for the custom resources that you handle,
+making them available to all of its clients.