README.md: drop invalid NotificationEmail parameter.
README.md: drop invalid NotificationEmail parameter.

Otherwise:

> A client error (ValidationError) occurred when calling the CreateStack operation: Parameters: [NotificationEmail] do not exist in the template

caremonkey/buildkite-aws-elastic-stack,buildkite/elastic-ci-stack-for-aws,caremonkey/buildkite-aws-elastic-stack,holmesjr/elastic-ci-stack-for-aws,holmesjr/elastic-ci-stack-for-aws