add and update changelog

Author: lkdvos

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+### Fixed
+
+### Changed
+
+- The `changebonds(state, ::RandExpand)` algorithm now no longer projects onto the nullspace
+  on both sides. This ensures that the expanded symmetry sectors can be selected beyond what
+  is allowed by two-site updates, which can be relevant for certain systems that have
+  symmetry-related obstructions.
+
+### Deprecated
+
+### Removed
+
+[unreleased]: https://github.com/quantumkithub/pepskit.jl/compare/v0.13.8...HEAD