Add a changelog

Add CHANGELOG.md and document the major changes from 0.1.0.

Author: gregoire-mullvad

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
+
+- The `Policy` type gained a full implementation of the quorum logic and the
+  ability to parse Sigsum policies.
+- There's a new `PolicyBuilder` type that can be used to programatically build
+  valid `Policy` values.
+- The crates now ships with the `sigsum-generic-2025-1` policy as well as three
+  test policies (same policies that are built into the reference Sigsum
+  tooling).
+
+### Removed
+
+- The `Policy::new_k_of_n()` constructor has been removed in favor of
+  `PolicyBuilder`.