Add CHANGELOG.md using the Keep a Changelog format (#444)

Since we started adding it only after 1.0 release, versions prior to v1.0.0 are untracked. I summarized all the changes in the v1.0.0 section instead.

Signed-off-by: Mihai Maruseac <[email protected]>

Author: mihaimaruseac

CHANGELOG.md

@@ -0,0 +1,28 @@
+# Changelog
+
+All notable changes to `model-signing` will be documented in this file.
+
+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).
+
+All versions prior to 1.0.0 are untracked.
+
+## [Unreleased]
+
+
+## [1.0.0] - 2024-04-04
+
+### Added
+- First stable release of `model_signing`.
+- Stable, backwards-compatible API.
+- Stable, clean, backwards-compatible CLI wrapping the API.
+- Well-defined signature scheme based on Sigstore bundle and DSSE envelope.
+- Well-defined, future proof, stable, in-toto format for the model signature.
+- Signing and verification using Sigstore, public/private key pairs, and signing certificates.
+- Support for signing/verification via scripts in the repo (`hatch run python -m model_signing`), installing the pip package (`python -m model_signing` or `model_signing`), or Docker containers that wrap around the CLI.
+- Support for Python 3.9, 3.10, 3.11, 3.12, and 3.13.
+- [Demo notebook](https://colab.sandbox.google.com/drive/18IB_uipduXYq0ohMxJv2xHfeihLIcGMT) to showcase API and CLI examples.
+
+
+[Unreleased]: https://github.com/sigstore/model-transparency/compare/v1.0.0...HEAD
+[1.0.0]: https://github.com/sigstore/model-transparency/compare/v0.1.0...v1.0.0