Add reference example for rights vocabulary desribing openAccess / restrictedAccess

[borsna]

On the rights element documentation:
https://datacite-metadata-schema.readthedocs.io/en/4.6/properties/rights/

OpenAire have some examples for data repositories for DataCite:
https://guidelines.openaire.eu/en/latest/data/field_rights.html#example

<rightsList>
  <rights rightsURI="info:eu-repo/semantics/openAccess" />
  <rights rightsURI="http://creativecommons.org/licenses/by/4.0/">
    Creative Commons Attribution 4.0 International
  </rights>
</rightsList>

If more repositories implement this (or some other vocabulary) it will be easier to get information if the resource is open or restricted.