add Carpentries linkref

Author: tobyhodges

CONTRIBUTING.md

@@ -136,6 +136,7 @@ happens on the [discussion mailing list][discuss-list],
 which everyone is welcome to join.
 You can also [reach us by email][contact].
 
+[c-site]: https://carpentires.org/
 [contact]: mailto:admin@software-carpentry.org
 [dc-issues]: https://github.com/issues?q=user%3Adatacarpentry
 [dc-lessons]: http://datacarpentry.org/lessons/