Add GLEX experiment to 'What warrants a changelog entry?'

doc/development/changelog.md

@@ -98,6 +98,7 @@ EE: true
   database records created during Cycle Analytics model spec."
 - _Any_ contribution from a community member, no matter how small, **may** have
   a changelog entry regardless of these guidelines if the contributor wants one.
+- Any [GLEX experiment](experiment_guide/gitlab_experiment.md) changes **should not** have a changelog entry.
 - [Removing](feature_flags/#changelog) a feature flag, when the new code is retained.
 
 ## Writing good changelog entries