release: Update CHANGELOG.md for 0.3.1

Author: jorisv

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.3.1] - 2023-12-21
+
 ### Fixed
 
 * Fixed a missing `#pragma once` in `linalg/bunchkaufman.hpp`
@@ -19,5 +21,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * This is the first release of `proxsuite-nlp`. This library is a joint effort between LAAS-CNRS and INRIA, and will be maintained and expanded in the future. Please provide constructive feedback and contribute!
 
-[Unreleased]: https://github.com/Simple-Robotics/proxsuite-nlp/compare/v0.3.0...HEAD
+[Unreleased]: https://github.com/Simple-Robotics/proxsuite-nlp/compare/v0.3.1...HEAD
+[0.3.1]: https://github.com/Simple-Robotics/proxsuite-nlp/compare/v0.3.0...v0.3.1
 [0.3.0]: https://github.com/Simple-Robotics/proxsuite-nlp/releases/tag/v0.3.0