Add link to ISO homepage (#306)

* Add link to ISO homepage

* Update docs/api/countries.rst

Co-authored-by: Daniel Huppmann <dh@dergelbesalon.at>

---------

Co-authored-by: Daniel Huppmann <dh@dergelbesalon.at>

docs/api/countries.rst

@@ -11,7 +11,7 @@ and model comparison.
 
 The :class:`nomenclature` package builds on the :class:`pycountry` package
 (`link <https://github.com/flyingcircusio/pycountry>`_) to provide a utility for country
-names based on the ISO 3166-1 standard.
+names based on the `ISO 3166-1 standard <https://en.wikipedia.org/wiki/ISO_3166-1>`_. 
 
 For consistency with established conventions in the modelling community, several country
 names are shortened compared to ISO 3166-1, e.g. from "Bolivia, Plurinational State of"