[AIRFLOW-730] Add Handshake to Airflow users

Closes apache#1972 from mhickman/master

README.md

@@ -105,6 +105,7 @@ Currently **officially** using Airflow:
 1. [Gentner Lab](http://github.com/gentnerlab) [[@neuromusic](https://github.com/neuromusic)]
 1. [Glassdoor](https://github.com/Glassdoor) [[@syvineckruyk](https://github.com/syvineckruyk)]
 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)]
 1. [HelloFresh](https://www.hellofresh.com) [[@tammymendt](https://github.com/tammymendt) & [@davidsbatista](https://github.com/davidsbatista) & [@iuriinedostup](https://github.com/iuriinedostup)]
 1. [Holimetrix](http://holimetrix.com/) [[@thibault-ketterer](https://github.com/thibault-ketterer)]