Updated documentation for connection using ALPN (#517)

ios/pubsub.md

@@ -89,6 +89,8 @@ Before you can publish/subscribe to a topic, you need to establish a connection.
 
 #### Certificate based mutual authentication
 
+To connect with the AWS IoT Core service on the standard MQTT port 8883, you can use the `connect` API as shown below.
+
 ```swift
 func mqttEventCallback(_ status: AWSIoTMQTTStatus ) {
     print("connection status = \(status.rawValue)")
@@ -100,8 +102,21 @@ iotDataManager.connect(withClientId: "<YOUR_CLIENT_ID>",
                        statusCallback: mqttEventCallback
 ```
 
+The AWS IoT Core service also allows you to connect devices using MQTT with certificate based mutual authentication on port 443. You can do this using the `connectUsingALPN` API as shown below. See [MQTT with TLS client authentication on port 443](https://aws.amazon.com/blogs/iot/mqtt-with-tls-client-authentication-on-port-443-why-it-is-useful-and-how-it-works/) for more information.
+
+```swift
+func mqttEventCallback(_ status: AWSIoTMQTTStatus ) {
+    print("connection status = \(status.rawValue)")
+}
+
+iotDataManager.connectUsingALPN(withClientId: "<YOUR_CLIENT_ID>",
+                       cleanSession: true,
+                       certificateId: "<YOUR_CERTIFICATE_ID>",
+                       statusCallback: mqttEventCallback
+```
+
 You can take a look at the [API Reference](https://aws-amplify.github.io/aws-sdk-ios/docs/reference/Classes/AWSIoTDataManager.html#//api/name/connectWithClientId:cleanSession:certificateId:statusCallback:
-) to know more information.
+) to get more information.
 
 #### AWS Credentials based Authentication