add link to AGENTS.md in CONTRIBUTING.md (open-telemetry#4460)

Assisted-by: Claude Sonnet 4.6

Author: MikeGoldsmith

CONTRIBUTING.md

@@ -17,6 +17,8 @@ Before you can contribute, you will need to sign the [Contributor License Agreem
 
 Please also read the [OpenTelemetry Contributor Guide](https://github.com/open-telemetry/community/blob/main/guides/contributor/README.md).
 
+If you are using AI agents to assist with contributions, please read [AGENTS.md](AGENTS.md) for guidance on how to use them responsibly in this project.
+
 ## Index
 
 - [Contributing to opentelemetry-python-contrib](#contributing-to-opentelemetry-python-contrib)