Add CHANGELOG links (#432)

Author: aomarks

CHANGELOG.md

@@ -1203,7 +1203,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Initial release.
 
-[unreleased]: https://github.com/google/playground-elements/compare/v0.18.1...HEAD
+[unreleased]: https://github.com/google/playground-elements/compare/v0.21.1...HEAD
+[0.21.1]: https://github.com/google/playground-elements/compare/v0.21.0...v0.21.1
+[0.21.0]: https://github.com/google/playground-elements/compare/v0.20.0...v0.21.0
+[0.20.0]: https://github.com/google/playground-elements/compare/v0.19.1...v0.20.0
+[0.19.1]: https://github.com/google/playground-elements/compare/v0.19.0...v0.19.1
+[0.19.0]: https://github.com/google/playground-elements/compare/v0.18.1...v0.19.0
 [0.18.1]: https://github.com/google/playground-elements/compare/v0.18.0...v0.18.1
 [0.18.0]: https://github.com/google/playground-elements/compare/v0.17.1...v0.18.0
 [0.17.1]: https://github.com/google/playground-elements/compare/v0.17.0...v0.17.1