Documentation: required IAM permissions. Needs iam:PassRole for ecsEventsRole

Hey there,

Just going through the setup process on this and found that IAM role for the actions user requires an additional permission.

> Error: User: arn:aws:iam::***:user/my-github-actions-user is not authorized to perform: iam:PassRole on resource: arn:aws:iam::***:role/ecsEventsRole

Just figured I'd pass along so it can be added to documentation.