Update changelog

Author: adamwathan

CHANGELOG.md

@@ -9,6 +9,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Nothing yet!
 
+## [2.0.0-alpha.21] - 2020-11-16
+
+### Changed
+
+- All custom properties have been internally namespaced under `tw-` to avoid collisions with end-user custom properties ([#2771](https://github.com/tailwindlabs/tailwindcss/pull/2771))
+
 ## [2.0.0-alpha.21] - 2020-11-15
 
 ### Changed
@@ -1210,7 +1216,8 @@ No release notes
 
 - Everything!
 
-[unreleased]: https://github.com/tailwindlabs/tailwindcss/compare/v2.0.0-alpha.21...HEAD
+[unreleased]: https://github.com/tailwindlabs/tailwindcss/compare/v2.0.0-alpha.22...HEAD
+[2.0.0-alpha.22]: https://github.com/tailwindlabs/tailwindcss/compare/v2.0.0-alpha.20...v2.0.0-alpha.22
 [2.0.0-alpha.21]: https://github.com/tailwindlabs/tailwindcss/compare/v2.0.0-alpha.20...v2.0.0-alpha.21
 [2.0.0-alpha.20]: https://github.com/tailwindlabs/tailwindcss/compare/v2.0.0-alpha.19...v2.0.0-alpha.20
 [2.0.0-alpha.19]: https://github.com/tailwindlabs/tailwindcss/compare/v2.0.0-alpha.18...v2.0.0-alpha.19