Document the repository as being in maintenance only (#1617)

* document the repository as being in maintenance only

* small wording changes

* fix wording

README.md

@@ -3,6 +3,15 @@
 [![Go Report Card](https://goreportcard.com/badge/go.elastic.co/apm/v2)](https://goreportcard.com/report/go.elastic.co/apm/v2)
 [![codecov.io](https://codecov.io/github/elastic/apm-agent-go/coverage.svg?branch=main)](https://codecov.io/github/elastic/apm-agent-go?branch=main)
 
+**NOTE**: This repository is in maintenance mode. Bug fixes will continue to be
+applied, but no new features will be implemented. To replace this agent, we
+recommend you to [migrate to the OpenTelemetry Go API and
+SDK](https://www.elastic.co/blog/elastic-go-apm-agent-to-opentelemetry-go-sdk),
+which provides similar features. In order to help you do a seamless migration,
+we recommend using our [OpenTelemetry
+Bridge](https://www.elastic.co/guide/en/apm/agent/go/current/opentelemetry.html).
+Please refer to the blog post above for further details.
+
 # apm-agent-go: APM Agent for Go
 
 This is the official Go package for [Elastic APM](https://www.elastic.co/solutions/apm).

docs/introduction.asciidoc

@@ -1,3 +1,12 @@
+*NOTE*: This repository is in maintenance mode. Bug fixes will continue to be
+applied, but no new features will be implemented. To replace this agent, we
+recommend you to
+https://www.elastic.co/blog/elastic-go-apm-agent-to-opentelemetry-go-sdk[migrate
+to the OpenTelemetry Go API and SDK], which provides similar features. In order
+to help you do a seamless migration, we recommend using our
+https://www.elastic.co/guide/en/apm/agent/go/current/opentelemetry.html[OpenTelemetry
+Bridge]. Please refer to the blog post above for further details.
+
 [[introduction]]
 == Introduction