docs: Add changelog file [skip ci]

Author: LukeSavefrogs

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/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [3.0.0] - 2023-06-08
+
+### Breaking changes
+
+- Renamed action output `jython-download-url` to `download-url`
+
+### Added
+
+- Support for Jython 2.0 and 2.1 (_only on `windows-*` runners_, see [#1](https://github.com/LukeSavefrogs/setup-jython/issues/1))
+- Exit with error when provided Jython version does not exist
+
+### Fixed
+
+- `download-url` description now do not have a placeholder text but actually holds meaningful info