Update the changelog

Author: fjarri

CHANGELOG.md

@@ -4,6 +4,16 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 
+## [0.5.1] - Unreleased
+
+### Fixed
+
+- Bumped `crypto-bigint` to 0.5.4. ([#35])
+
+
+[#35]: https://github.com/nucypher/rust-umbral/pull/35
+
+
 ## [0.5.0] - 2023-08-20
 
 ### Changed