Update KV READMEs (Azure#25928)

Author: jeesang7

sdk/keyvault/azure-keyvault-certificates/README.md

@@ -60,6 +60,7 @@ This section contains code snippets covering common tasks:
 * [Update properties of an existing certificate](#update-properties-of-an-existing-certificate)
 * [Delete a certificate](#delete-a-certificate)
 * [List properties of certificates](#list-properties-of-certificates)
+* [Async operations](#async-operations)
 * [Asynchronously create a certificate](#asynchronously-create-a-certificate)
 * [Asynchronously list properties of certificates](#asynchronously-list-properties-of-certificates)
 

sdk/keyvault/azure-keyvault-secrets/README.md

@@ -71,6 +71,7 @@ This section contains code snippets covering common tasks:
 * [Update secret metadata](#update-secret-metadata)
 * [Delete a secret](#delete-a-secret)
 * [List secrets](#list-secrets)
+* [Async API](#async-api)
 * [Asynchronously create a secret](#asynchronously-create-a-secret)
 * [Asynchronously list secrets](#asynchronously-list-secrets)