[AIRFLOW-XXX] Add DoorDash to README.md (#3980)

DoorDash uses Airflow https://softwareengineeringdaily.com/2018/09/28/doordash/

Author: qifengzhou

README.md

@@ -148,6 +148,7 @@ Currently **officially** using Airflow:
 1. [DataFox](https://www.datafox.com/) [[@sudowork](https://github.com/sudowork)]
 1. [Digital First Media](http://www.digitalfirstmedia.com/) [[@duffn](https://github.com/duffn) & [@mschmo](https://github.com/mschmo) & [@seanmuth](https://github.com/seanmuth)]
 1. [DocuTAP](https://www.docutap.com/) [[@jshvrsn](https://github.com/jshvrsn) & [@lhvphan](https://github.com/lhvphan) & [@cloneluke](https://github.com/cloneluke)]
+1. [DoorDash](https://www.doordash.com/)
 1. [Dotmodus](http://dotmodus.com) [[@dannylee12](https://github.com/dannylee12)]
 1. [Drivy](https://www.drivy.com) [[@AntoineAugusti](https://github.com/AntoineAugusti)]
 1. [Easy Taxi](http://www.easytaxi.com/) [[@caique-lima](https://github.com/caique-lima) & [@WesleyBatista](https://github.com/WesleyBatista) & [@diraol](https://github.com/diraol)]