Clarify recommendations around OCI annotations

[jlegrone]

In https://github.com/cnabio/cnab-spec/blob/master/201-representing-CNAB-in-OCI.md, we discuss how bundles should be represented as OCI manifests. There is one mention of annotations:

While implementations MAY choose to surface information from the bundle as top-level annotations in the OCI representations, they MUST store the unmodified bundle file as a blob.

I think it would be useful to clarify the following points, along with an updated example manifest.

1. How additional bundle metadata SHOULD be reflected in annotations, including which fields of a bundle.json should map to which pre-defined annotations
2. Should annotations go on the top level of the manifest object, or under .config?