[AIRFLOW-854] Add OKI as Airflow user

OKI == Open Knowledge International

Closes apache#2061 from vitorbaptista/patch-1

README.md

@@ -131,6 +131,7 @@ Currently **officially** using Airflow:
 1. [Nerdwallet](https://www.nerdwallet.com)
 1. [OfferUp](https://offerupnow.com)
 1. [OneFineStay](https://www.onefinestay.com) [[@slangwald](https://github.com/slangwald)]
+1. [Open Knowledge International](https://okfn.org) [@vitorbaptista](https://github.com/vitorbaptista)
 1. [PayPal](https://www.paypal.com/) [[@jhsenjaliya](https://github.com/jhsenjaliya)]
 1. [Postmates](http://www.postmates.com) [[@syeoryn](https://github.com/syeoryn)]
 1. [Qubole](https://qubole.com) [[@msumit](https://github.com/msumit)]