chore: updating advice on message processing (#366)

README.md

@@ -55,6 +55,7 @@ app.start();
 - Messages are deleted from the queue once the handler function has completed successfully.
 - Throwing an error (or returning a rejected promise) from the handler function will cause the message to be left on the queue. An [SQS redrive policy](https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/SQSDeadLetterQueue.html) can be used to move messages that cannot be processed to a dead letter queue.
 - By default messages are processed one at a time – a new message won't be received until the first one has been processed. To process messages in parallel, use the `batchSize` option [detailed below](#options).
+- By default, messages that are sent to the `handleMessage` and `handleMessageBatch` functions will be considered as processed if they return without an error. To acknowledge individual messages, please return the message that you want to acknowledge if you are using `handleMessage` or the messages for `handleMessageBatch`. It's also important to await any processing that you are doing to ensure that messages are processed one at a time.
 
 ### Credentials