document Alias usage as ARN

[doebrowsk]

The following information needs to be documented correctly in the readme:
The Alias field for this store type is used to hold the ARN of a certificate. Inventory jobs will return the ARN of existing certificates. When adding a certificate, unless it is intended as a renewal or to replace an existing certificate, the Alias should not be specified. When doing a replacement or renewal, only user-imported certificates can be replaced by specifying the existing ARN of the certificate that should be replaced for the Management job.