add expectations for new CHANGELOG entries

Author: ezhang6811

CHANGELOG.md

@@ -4,4 +4,10 @@ All notable changes to this project will be documented in this file.
 
 > **Note:** This CHANGELOG was created starting from version 0.12.0. Earlier changes are not documented here.
 
+For any change that affects end users of this package, please add an entry under the **Unreleased** section. Briefly summarize the change and provide the link to the PR. Example:
+- add GenAI attribute support for Amazon Bedrock models
+  ([#300](https://github.com/aws-observability/aws-otel-python-instrumentation/pull/300))
+
+If your change does not need a CHANGELOG entry, add the "skip changelog" label to your PR.
+
 ## Unreleased