Add Appian to the list of Orchestrator users.

Author: MatthewMiller2

docs/users.md

@@ -10,6 +10,7 @@ you would like to be added please send a pull request to github.com/openark/orch
 Please keep this list in alphabetical order and include the company name and url and
 a short description of usage.
 
+* [Appian](https://appian.com/)
 * [Booking.com](http://www.booking.com)
 * [Box](http://www.box.com)
 * [Citymobil](https://www.city-mobil.ru)