Governance: Add policy for documentation standards in external container registries

[darach]

Tremor uses a number of external repository / registry services so that tremor is discoverable.

Examples are docker hub, the vscode extension registry, the azure registry, amongst others.

We should document best practices and guidelines for documentation in each of these
repository formats so that we conform to registry / repository best practice and so that
tremor is uniformly and consistently discoverable across them.

Community, please feel free to add comments, suggestions or references to existing policies
and standards we could clone, adopt or tune to suit the needs of the project.