Update README to reflect production readiness

Author: marckohlbrugge

README.md

@@ -4,7 +4,7 @@
 
 Open-source email observability for AWS SES by [Marc Köhlbrugge](https://x.com/marckohlbrugge).
 
-> **Note:** Sessy is still in active development and not quite ready for production use. Feel free to have a look around and follow along as I build it.
+> **Note:** Sessy is already running in production at [BetaList](https://betalist.com), but it's still in active development. Expect breaking changes, database migrations, and evolving APIs. That said, feel free to start using it!
 
 ## What is Sessy?