Add definitions/examples on how to use identifiers

We should provide definitions (or better examples) which character string we expect to be used for identifiers. For example, ORCID has the operable URI "http://orcid.org/ID" and the plain "ID". IGSN has three representations that could be an identifier (the URI, the handle, the ID itself) and a DOI has two representations (the URI, the handle).
