Update CHANGELOG.md after release (#441)

Author: anders-kiaer

CHANGELOG.md

@@ -6,6 +6,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [UNRELEASED] - YYYY-MM-DD
 
+## [0.3.1] - 2021-04-29
+
 ### Fixed
 - [#429](https://github.com/equinor/webviz-config/pull/429) - Moved `FutureWarnings`
 from `deprecated_decorators.py` to `_config_parser.py`. Simplified deprecation warnings.