Add definitions/examples on how to use identifiers

[ulbricht]

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).

[kitchenprinzessin3880]

+1 ( i have also stated this here, see point2 @ [https://github.com//issues/8]