Update changelog

Author: adamwathan

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+## Changed
+
+- Use `rem` units for breakpoints by default instead of `px` ([#13469](https://github.com/tailwindlabs/tailwindcss/pull/13469))
 
 ## [4.0.0-alpha.14] - 2024-04-09