Add Changelog (#25)

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file. For help with updating to new
+PyO3 versions, please see the [migration guide](https://pyo3.rs/latest/migration.html).
+
+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).
+
+To see unreleased changes, please see the CHANGELOG on the main branch.
+
+<!-- towncrier release notes start -->
+
+## [Unreleased]
+
+- Bump minimum version of `pyo3` dependency to 0.23. [#21](https://github.com/PyO3/pyo3-async-runtimes/pull/21)
+
+## [0.22.0] - 2024-10-28
+
+- Move from [the `davidhewitt/pyo3-asyncio` fork](https://github.com/davidhewitt/pyo3-asyncio) (had been published as `pyo3-asyncio-0.21`) to `pyo3-async-runtimes`.
+- Bump minimum version of `syn` dependency to 2. [#12](https://github.com/PyO3/pyo3-async-runtimes/pull/12)
+
+## Older versions
+
+Previous versions were published from [`pyo3-asyncio`](https://github.com/awestlake87/pyo3-asyncio). Consult that library for older changes.
+
+[Unreleased]: https://github.com/PyO3/pyo3-async-runtimes/compare/v0.22.0...HEAD
+[0.22.0]: https://github.com/PyO3/pyo3-async-runtimes/tree/0.22.0