add link references

_includes/links.md

@@ -48,6 +48,7 @@
 [ford-any-colour-black]: https://en.wikiquote.org/wiki/Talk:Henry_Ford#Any_color,_as_long_as_it_is_black
 [fstring]: https://realpython.com/python-f-strings/#f-strings-a-new-and-improved-way-to-format-strings-in-python
 [functools-module]: https://docs.python.org/3/library/functools.html
+[gapminder]: https://www.gapminder.org/
 [generator-delegate]: https://docs.python.org/3/whatsnew/3.3.html#pep-380
 [github-blog-contributing]: https://github.blog/2012-09-17-contributing-guidelines/
 [github-importer]: https://import.github.com/
@@ -151,6 +152,7 @@
 [sphinx-docstring-format]: https://sphinx-rtd-tutorial.readthedocs.io/en/latest/docstrings.html#
 [sphinx]: https://www.sphinx-doc.org
 [styles]: https://github.com/carpentries/styles/
+[swc-gapminder-data]: http://swcarpentry.github.io/python-novice-gapminder/setup/#getting-the-data
 [swc-git]: https://swcarpentry.github.io/git-novice/
 [swc-lessons]: https://software-carpentry.org/lessons/
 [swc-python-gapminder]: http://swcarpentry.github.io/python-novice-gapminder/