add manual puback to main readme

Author: sbSteveK

README.md

@@ -24,6 +24,7 @@ The primary purpose of the AWS IoT Device SDK for JavaScript v2 is to simplify t
 * Integrated service clients for AWS IoT Core services
 * Secure device connections to AWS IoT Core using MQTT protocol including MQTT 5.0
 * Support for [multiple authentication methods and connection types](./documents/MQTT5_Userguide.md#how-to-create-an-mqtt5-client-based-on-desired-connection-method)
+* Support for [manual publish acknowledgement](./documents/MQTT5_Userguide.md#manual-publish-acknowledgement) for control over QoS 1 PUBACK delivery
 
 #### Supported AWS IoT Core services