docs: update changelog for release

Author: SimenB

CHANGELOG.md

@@ -13,6 +13,10 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
+## [21.4.0] - 2017-12-04
+
+### Added
+
 * Add `no-large-snapshots` rule
   ([#24](https://github.com/jest-community/eslint-plugin-jest/pull/24))
 * Add `prefer-to-be-null` rule
@@ -44,7 +48,8 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 Initial fork from the Jest repo
 
-[unreleased]: https://github.com/jest-community/eslint-plugin-jest/compare/v21.3.2...HEAD
+[unreleased]: https://github.com/jest-community/eslint-plugin-jest/compare/v21.4.0...HEAD
+[21.4.0]: https://github.com/jest-community/eslint-plugin-jest/compare/v21.3.2...v21.4.0
 [21.3.2]: https://github.com/jest-community/eslint-plugin-jest/compare/v21.3.1...v21.3.2
 [21.3.1]: https://github.com/jest-community/eslint-plugin-jest/compare/v21.3.0...v21.3.1
 [21.3.0]: https://github.com/jest-community/eslint-plugin-jest/compare/b38a897f164e91fc78ab166529e031d76fc5ca70...v21.3.0