Update changelog

Author: adamwathan

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Add `flex-basis` utilities ([#5671](https://github.com/tailwindlabs/tailwindcss/pull/5671))
+- Make negative values a first-class feature ([#5709](https://github.com/tailwindlabs/tailwindcss/pull/5709))
 
 ## [3.0.0-alpha.1] - 2021-10-01