docs: prepare v0.3.1

Author: Rogdham

CHANGELOG.md

@@ -5,9 +5,9 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/), and this project
 adheres to [Semantic Versioning](https://semver.org/).
 
-## [Unreleased]
+## [0.3.1] - 2021-12-26
 
-[unreleased]: https://github.com/rogdham/bigxml/compare/v0.3.0...HEAD
+[0.3.1]: https://github.com/rogdham/python-xz/releases/tag/v0.3.1
 
 ### :house: Internal