docs: Add initial changelog, based on the keepachangelog format

Author: faern

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](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+### Categories each change fall into
+
+* **Added**: for new features.
+* **Changed**: for changes in existing functionality.
+* **Deprecated**: for soon-to-be removed features.
+* **Removed**: for now removed features.
+* **Fixed**: for any bug fixes.
+* **Security**: in case of vulnerabilities.
+
+
+## [Unreleased]
+
+
+## [0.2.0] - 2025-05-11
+This release is identical to `0.1.2`. This is re-released as `0.2.0` since the changes
+were semver breaking.
+
+
+## [0.1.2] - 2025-05-10
+### Added
+- Use `const`-compatible constructors in `ipnetwork`, allowing using the macros in const
+  contexts.
+
+### Changed
+- Disable default features on `ipnetwork` dependency. Strips out `serde` as a
+  transitive dependency.
+- Upgrade `ipnetwork` dependency from `0.20` to `0.21`. This is a breaking change
+  since the `ipnetwork` dependency is part of the public API of this library.