Add CHANGELOG entry for external ID validation change

Author: jbonhag

CHANGELOG.md

@@ -169,6 +169,7 @@
 ## Enhancements
 
 * Adds BETA support for `Stacks` resources, which is EXPERIMENTAL, SUBJECT TO CHANGE, and may not be available to all users by @brandonc. [#920](https://github.com/hashicorp/go-tfe/pull/920)
+* Adds support for managing resources that have HCP IDs by @roncodingenthusiast. [#924](https://github.com/hashicorp/go-tfe/pull/924)
 
 # v1.57.0