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!
+### Added
+
+- Add `flex-basis` utilities ([#5671](https://github.com/tailwindlabs/tailwindcss/pull/5671))
 
 ## [3.0.0-alpha.1] - 2021-10-01