Add initial changelog

Author: garyb

CHANGELOG.md

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+# Changelog
+
+Notable changes to this project are 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]
+
+Breaking changes:
+
+New features:
+
+Bugfixes:
+
+Other improvements:
+
+
+## [v1.0.0](https://github.com/purescript-contrib/purescript-js-promise-aff/releases/tag/v1.0.0) - 2022-11-09
+
+Initial release, forked from `purescript-aff-promise` and intended as its replacement from now on.