SimpleIdentifier: Explain how to set scheme

[duncandewhurst]

The SimpleIdentifier subschema is referenced in identifiers, which is used to provide internal identifiers for projects. However, no guidance is provided on how to set .scheme when disclosing an internal identifier:

The example suggests that it should be set to "internal":

However, its possible that the projects in an OC4IDS dataset might have identifiers drawn from more than one internal scheme, so I think it would be better to reuse the suggestion from SimpleIdentifier in OCDS 1.2:

The list, register or system from which the identifier is taken. If the scope of the list, register or system from which the identifier is drawn is national, it is suggested to follow the pattern {ISO 3166-1 alpha-2}-{system}. If it is subnational, it is suggested to follow the pattern {ISO 3166-2}-{system}.