Add list of notary annotations

[SteveLasker]

Notary adds several annotations. Would it help to have a single markdown file that lists the annotations, and defines the format? For instance, we've followed a camelCase formatting of annotations. To avoid new annotations from diverging or new annotations that are duplicative of existing ones, should we document these?

[sajayantony]

I would recommend adding a section to this https://github.com/notaryproject/notaryproject/blob/main/signature-specification.md#storage with annotations.

Also, it is currently a bit confusing since there are annotations in the Payload and annotations in the manifest https://github.com/notaryproject/notaryproject/blob/main/signature-envelope-jws.md#storage

For all purposes my understanding is that registries aren't expected to look at the payload as currently registries and clients only look at the manifest at the lowest level.