[ADD] initialize changelog

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/1.0.0/) (+ the Migration Guide section),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [unreleased] 
+
+Forth unofficial release of NGMT for testing purposes.
+
+### Fixed
+- Gait sequence detection with datetime [[FIX]](https://github.com/neurogeriatricskiel/NGMT/commit/ad102772fd355e6ca64f7ad6a00a4c95d066a1d6)
+
+## [0.0.3] - 2024-02-27
+
+Third unofficial release of NGMT for testing purposes.
+
+### Added
+- Pyarrow as dependency [[ADD]](https://github.com/neurogeriatricskiel/NGMT/commit/22e401a5519cc9adde37b5c752a361a07d8166ac)
+- Testing coverage [[ADD]](https://github.com/neurogeriatricskiel/NGMT/commit/f6a919100e7a9d7319a4af77592a78bd6949bb69)
+
+### Fixed
+- Existing algorithms adapted to new dataclass structure [[FIX]](https://github.com/neurogeriatricskiel/NGMT/commit/3adf7756d9998b36454dccc86d9e2283200d72ed)
+
+## [0.0.2] - 2024-01-22
+
+Second unofficial release of NGMT for testing purposes.
+
+### Added
+- Physical acitivity monitoring algorithm [[#29]](https://github.com/neurogeriatricskiel/NGMT/commit/a8d9067cde00f0c9a0dba8b7fc623ba4eeb32d0a)
+
+## [0.0.1] - 2023-11-21
+
+This is the first unofficial release of NGMT.
+Therefore, we do not have a proper changelog for this release.
+
+### Added
+- All the things :)
+
+### Changed
+
+### Deprecated
+
+### Removed
+
+### Fixed
+
+### Migration Guide