Document how to display information from your app

[Doug-AWS]

Replaces this issue created by Doug-AWS on Feb 28 in the CDK repo: aws/aws-cdk#1908.

Original description:

Something like

sys.out("The S3 bucket name is " + myBucket.name

Feedback from user:

I found it difficult to discover how to add outputs to the stack (cdk.Output...), and then how to get the output values from the CDK CLI. I expected it to be part of the "Concepts" hierarchy, possibly something like "Passing out Data" to go along with "Passing in Data".