message: add American Express to list of users on readme
diff: @@ -171,6 +171,7 @@ the world know they are using Superset. Join our growing community!
  1. [Airbnb](https://github.com/airbnb)
  1. [Airboxlab](https://foobot.io)
  1. [Aktia Bank plc](https://www.aktia.com)
  + 1. [American Express](https://www.americanexpress.com)
  1. [Amino](https://amino.com)
  1. [Apollo GraphQL](https://www.apollographql.com/)
  1. [Ascendica Development](http://ascendicadevelopment.com)
  