Update CHANGELOG.md

Author: adamwathan

CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Replace `chalk` with `picocolors` ([#6039](https://github.com/tailwindlabs/tailwindcss/pull/6039))
 - Replace `cosmiconfig` with `lilconfig` ([#6039](https://github.com/tailwindlabs/tailwindcss/pull/6038))
+- Update `cssnano` to avoid removing empty variables when minifying ([#7818](https://github.com/tailwindlabs/tailwindcss/pull/7818))
 
 ### Added