Add credits section.

README.md

@@ -105,6 +105,10 @@ The long-term goal of this project is to retire itself once App Store Connect fi
 
 Once the App Store Connect APIs support webhooks, if a 3rd party app is still needed to receive those webhooks and post to Slack (i.e. if Apple doesn't provide one), then this project will be updated to provide those features as well.
 
+# Credits
+
+This app is heavily based on [@erikvillegas](https://github.com/erikvillegas)'s [itunes-connect-slack](https://github.com/erikvillegas/itunes-connect-slack). Most of the credit goes to him for figuring out the integration between ruby and javascript.
+
 # License
 
 This project is open source and covered by a standard 2-clause BSD license. That means you have to mention *Roger Oba* as the original author of this code and reproduce the LICENSE text inside your app, repository, project or research paper.