[AIRFLOW-2378] Add Groupon to list of current users

Closes #3267 from stevencasey/add_groupon

README.md

@@ -140,6 +140,7 @@ Currently **officially** using Airflow:
 1. [GovTech GDS](https://gds-gov.tech) [[@chrissng](https://github.com/chrissng) & [@datagovsg](https://github.com/datagovsg)]
 1. [Grand Rounds](https://www.grandrounds.com/) [[@richddr](https://github.com/richddr), [@timz1290](https://github.com/timz1290), [@wenever](https://github.com/@wenever), & [@runongirlrunon](https://github.com/runongirlrunon)]
 1. [Groupalia](http://es.groupalia.com) [[@jesusfcr](https://github.com/jesusfcr)]
+1. [Groupon](https://groupon.com) [[@stevencasey](https://github.com/stevencasey)]
 1. [Gusto](https://gusto.com) [[@frankhsu](https://github.com/frankhsu)]
 1. [Handshake](https://joinhandshake.com/) [[@mhickman](https://github.com/mhickman)]
 1. [Handy](http://www.handy.com/careers/73115?gh_jid=73115&gh_src=o5qcxn) [[@marcintustin](https://github.com/marcintustin) / [@mtustin-handy](https://github.com/mtustin-handy)]