docs: update users list (argoproj#1307)

Signed-off-by: Ardi Mehist <[email protected]>

Author: omgapuppy

USERS.md

@@ -17,4 +17,5 @@ Organizations below are **officially** using Argo Rollouts. Please send a PR wit
 1. [Quizlet](https://quizlet.com)
 1. [Skillz](https://www.skillz.com)
 1. [PayPal](https://www.paypal.com/)
-1. [Shipt](https://www.shipt.com/)
+1. [Shipt](https://www.shipt.com/)
+1. [Nitro](https://www.gonitro.com)