Update README to explain deployment process clearly

[drumadrian]

This file: https://github.com/awslabs/distributed-load-testing-on-aws/blob/master/deployment/distributed-load-testing-on-aws.yaml

includes the following Bucket and Key Mapping:

SourceCode: General: S3Bucket: CODE_BUCKET KeyPrefix: SOLUTION_NAME/CODE_VERSION

In the template published here: https://aws.amazon.com/solutions/distributed-load-testing-on-aws/?did=sl_card&trk=sl_card

that is hosted at this path: https://s3.amazonaws.com/solutions-reference/distributed-load-testing-on-aws/latest/distributed-load-testing-on-aws.template

The Mapping is this:

SourceCode: General: S3Bucket: solutions KeyPrefix: distributed-load-testing-on-aws/v1.0.0

This should be explained in more detail so a contributor planning to build and improve this solution knows that the template needs to be updated with the Bucket and Key of the new owner's AWS account.