Add caveat for AWS Backup: Cross-account sharing must be enabled in the Management Account

[kmcquade]

https://docs.aws.amazon.com/aws-backup/latest/devguide/manage-cross-account.html#enable-cross-account

It is not possible to automatically share it with other accounts just by modifying the resource policy. Thought I would put a note of that here, in case someone wanted to add it in the README