Update CHANGELOG.md

CHANGELOG.md

@@ -6,4 +6,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [2.0.0] - 2023-03-09
 ### Breaking Changes
-- `postmen-api-key` is now deprecated and cannot be used for authentication. Please use this [guide](https://www.aftership.com/docs/shipping/quickstart/api-quick-start#2-get-the-api-key) to obtain the 'as-api-key' for making requests with this version. **This change does not affect the historical usage of SDK.**
+- Legacy API key is now deprecated and cannot be used for authentication. Please use this [guide](https://www.aftership.com/docs/shipping/quickstart/api-quick-start#2-get-the-api-key) to obtain a modern API key for making requests with this version. 
+
+**This update does not require any changes to your code base. However the legacy Postmen API key cannot be used anymore.**
+
+You can check which version of API keys you are using by looking at the header key used for authentication:
+
+Type | API Header Key
+--- | ---
+Legacy API Key | `postmen-api-key`
+Modern API Key | `as-api-key`