[AIRFLOW-599] Adding spotify to Airflow users

Dear Airflow Maintainers,

Please accept this PR that addresses the following
issues:
- Adds Spotify to list of Airflow users

Closes apache#1855 from znichols/spotify_use

README.md

@@ -125,6 +125,7 @@ Currently **officially** using Airflow:
 1. [Sidecar](https://hello.getsidecar.com/) [[@getsidecar](https://github.com/getsidecar)]
 1. [SimilarWeb](https://www.similarweb.com/) [[@similarweb](https://github.com/similarweb)]
 1. [SmartNews](https://www.smartnews.com/) [[@takus](https://github.com/takus)]
+1. [Spotify](https://github.com/spotify) [[@znichols](https://github.com/znichols)]
 1. Stripe [[@jbalogh](https://github.com/jbalogh)]
 1. [Thumbtack](https://www.thumbtack.com/) [[@natekupp](https://github.com/natekupp)]
 1. [T2 Systems](http://t2systems.com) [[@unclaimedpants](https://github.com/unclaimedpants)]