Added note about configuring aws cli

The deploy script depends on AWS being configured with credentials and it doesn't report it's failure if AWS hasn't been configured.  This seemingly obvious step might not be intuitive to someone without prior experience using aws cli

Author: levous

README.md

@@ -33,7 +33,7 @@ image will be served from S3 directly.
     CloudFormation.
 
     The deployment script requires the [AWS CLI][cli] version 1.11.19 or newer
-    to be installed.
+    to be installed.  Be sure to [set your AWS credentials][aws-configure] using `aws configure`
 
 3. Test the function
 
@@ -64,3 +64,4 @@ This reference architecture sample is [licensed][license] under Apache 2.0.
 [sharp]: https://github.com/lovell/sharp
 [amazon-linux]: https://aws.amazon.com/blogs/compute/nodejs-packages-in-lambda/
 [cli]: https://aws.amazon.com/cli/
+[aws-configure]: https://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html