docs: Deprecation message

README.md

@@ -4,6 +4,13 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/getsentry/raven-go)](https://goreportcard.com/report/github.com/getsentry/raven-go)
 [![GoDoc](https://godoc.org/github.com/getsentry/raven-go?status.svg)](https://godoc.org/github.com/getsentry/raven-go)
 
+---
+
+> The `raven-go` SDK is no longer maintained and was superseded by the `sentry-go` SDK.
+> Learn more about the project on [GitHub](https://github.com/getsentry/sentry-go) and check out the [migration guide](https://docs.sentry.io/platforms/go/migration/).
+
+---
+
 raven is the official Go SDK for the [Sentry](https://github.com/getsentry/sentry)
 event/error logging system.