Update changelog

Author: thecrypticace

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Allow default ring color to be a function ([#7587](https://github.com/tailwindlabs/tailwindcss/pull/7587))
 - Add `rgb` and `hsl` color helpers for CSS variables ([#7665](https://github.com/tailwindlabs/tailwindcss/pull/7665))
+- Support PostCSS `Document` nodes ([#7291](https://github.com/tailwindlabs/tailwindcss/pull/7291))
 
 ## [3.0.23] - 2022-02-16