Adding GAZ-countries (ready to merge) (#2284)

* Adding GAZ-countries (DO NOT MERGE YET)

See:

- #2269

Do not merge until the PURL is available:

- OBOFoundry/purl.obolibrary.org#897

* Update ontology/gaz.md

Co-authored-by: Nico Matentzoglu <nicolas.matentzoglu@gmail.com>

---------

Co-authored-by: Nico Matentzoglu <nicolas.matentzoglu@gmail.com>

Author: cmungall

ontology/gaz.md

@@ -21,6 +21,9 @@ mailing_list: https://groups.google.com/forum/#!forum/obo-gazetteer
 products:
 - id: gaz.owl
 - id: gaz.obo
+- id: gaz/gaz-countries.owl
+  title: GAZ countries
+  description: A country specific subset of the GAZ.
 repository: https://github.com/EnvironmentOntology/gaz
 tracker: https://github.com/EnvironmentOntology/gaz/issues
 activity_status: inactive