Update changelog with 0.4.0 release

Author: joelostblom

CHANGELOG.md

@@ -5,7 +5,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 Version number is based on [Semver](https://semver.org/).
 Note that major version zero (0.y.z) is for initial development and anything may change at any time.
 
-## [Unreleased]
+## [0.4.0] - 2022-04-30
 
 ### Added
 - Show mean and median grade in plot.