Update docs to reflect us-east-1 limitation due to restriction on using ACM and CloudFront

[cloudkj]

As per https://docs.aws.amazon.com/acm/latest/userguide/acm-regions.html, ACM has a restriction that requires certificates to be created in the us-east-1 region in order to be used with CloudFront:

To use an ACM Certificate with Amazon CloudFront, you must request or import the certificate in the US East (N. Virginia) region. ACM Certificates in this region that are associated with a CloudFront distribution are distributed to all the geographic locations configured for that distribution.

The docs should be updated to reflect this limitation. It's possible that CloudFormation Stack Sets can offer a path forward by requesting only the ACM certificate in us-east-1 but other resources in the chosen region, but some work is needed to see if that's possible.

See #9 (comment)