Add maintenance mode notice to README

README.md

@@ -12,6 +12,16 @@
 
 Node.js Support for StackDriver Trace
 
+> **Warning**
+>
+> `cloud-trace-nodejs` is in maintenance mode. This means that we'll continue to fix bugs
+> add add security patches. We'll consider merging new feature contributions (depending
+> on the anticipated maintenance cost). But we won't develop new features ourselves.
+>
+> In particular, we will not add support for new major versions of libraries.
+>
+> We encourage users to migrate to [OpenTelemetry JS Instrumentation][otel] instead.
+
 
 A comprehensive list of changes in each version may be found in
 [the CHANGELOG](https://github.com/googleapis/cloud-trace-nodejs/blob/main/CHANGELOG.md).
@@ -292,3 +302,4 @@ See [LICENSE](https://github.com/googleapis/cloud-trace-nodejs/blob/main/LICENSE
 [billing]: https://support.google.com/cloud/answer/6293499#enable-billing
 [enable_api]: https://console.cloud.google.com/flows/enableapi?apiid=cloudtrace.googleapis.com
 [auth]: https://cloud.google.com/docs/authentication/getting-started
+[otel]: https://opentelemetry.io/docs/instrumentation/js/libraries/