guidelines on annotations for metadata

[ryandawsonuk]

One way to track metadata related to a model would be to add annotations to the SeldonDeployment. We've talked about other ways but annotations would be very flexible and allow for recording references to external metadata stores. If we do go this way we should provide guidelines to avoid everyone making up their own annotations with different wording and spellings. Alternatively, we could put specific named fields on the custom resource.