Add the changelog

CHANGELOG.md

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+# Changelog
+
+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).
+
+
+## [Unreleased]
+
+### Changed
+
+- `SecretKey` and `SecretKeyFactory` no longer implement `SerializableToArray`, but implement `SerializableToSecretArray` instead. Correspondingly, in the bindings these objects implement `to_secret_bytes()` instead of `__bytes__()` (for Python), and `toSecretBytes()` instead of `toBytes()` (for WASM). ([#53])
+- `SecretKey`, `SecretKeyFactory` and `Signer` do not implement `PartialEq` anymore. Corresponding methods in the bindings were removed as well. ([#53])
+- Bumped `k256` to `0.9` and `ecdsa` to `0.12.2`. ([#53])
+
+
+### Added
+
+- `SecretBox` struct, a wrapper making operations with secret data explicit and ensuring zeroization on drop ([#53])
+
+
+### Fixed
+
+- Multiple internal changes for safe secret data handling using `SecretBox` ([#53])
+
+
+[#53]: https://github.com/nucypher/rust-umbral/pull/53
+
+
+## [0.2.0] - 2021-06-14
+
+- Initial release.
+
+[Unreleased]: https://github.com/nucypher/rust-umbral/compare/v0.2.0...HEAD
+[0.2.0]: https://github.com/nucypher/rust-umbral/releases/tag/v0.2.0