[AIRFLOW-943] Update Digital First Media in users list

- Fix incorrect link
- Add new users

There is no JIRA ticket for this change nor any
tests involved.

Closes apache#2115 from duffn/patch-2

README.md

@@ -101,7 +101,7 @@ Currently **officially** using Airflow:
 1. [Clover Health](https://www.cloverhealth.com) [[@gwax](https://github.com/gwax) & [@vansivallab](https://github.com/vansivallab)]
 1. Chartboost [[@cgelman](https://github.com/cgelman) & [@dclubb](https://github.com/dclubb)]
 1. [Cotap](https://github.com/cotap/) [[@maraca](https://github.com/maraca) & [@richardchew](https://github.com/richardchew)]
-1. [Digital First Media](https://www.digitalfirstmedia.com/) [[@duffn](https://github.com/duffn)]
+1. [Digital First Media](http://www.digitalfirstmedia.com/) [[@duffn](https://github.com/duffn) & [@mschmo](https://github.com/mschmo) & [@seanmuth](https://github.com/seanmuth)]
 1. [Easy Taxi](http://www.easytaxi.com/) [[@caique-lima](https://github.com/caique-lima) & [@WesleyBatista](https://github.com/WesleyBatista)]
 1. [FreshBooks](https://github.com/freshbooks) [[@DinoCow](https://github.com/DinoCow)]
 1. [Gentner Lab](http://github.com/gentnerlab) [[@neuromusic](https://github.com/neuromusic)]