[skip ci] prepare for v0.4.0 release

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
+## [v0.4.0]
 - Added kid and alg to `db` package
 - Updated loading cipher options
 
@@ -93,7 +95,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Initial creation
 - Created `db` and `xvault` package
 
-[Unreleased]: https://github.com/Comcast/codex/compare/v0.3.3...HEAD
+[Unreleased]: https://github.com/Comcast/codex/compare/v0.4.0...HEAD
+[v0.4.0]: https://github.com/Comcast/codex/compare/v0.3.3...v0.4.0
 [v0.3.3]: https://github.com/Comcast/codex/compare/v0.3.2...v0.3.3
 [v0.3.2]: https://github.com/Comcast/codex/compare/v0.3.1...v0.3.2
 [v0.3.1]: https://github.com/Comcast/codex/compare/v0.3.0...v0.3.1