AWS SAR vulnerability in README.md

[johspaeth]

Hi @dacort

the README states that deployment is done using AWS SAR. There has been a recent cross-account vulnerability related to AWS SAR and your repo may be affected. In the blog post, you find an explanation, as well as a short explanation of what needs to be fixed.

As your README deployment contains the vulnerability, I suggest updating it. To prevent the vulnerability, you need to add a block

            Condition:
              StringEquals:
                "aws:SourceAccount":  <AWS::AccountId>