Add CHANGELOG.md

Create a CHANGELOG based on keepachangelog standard for keeping track
of changes.

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]
+
+## [0.1.0] - 2022-06-08
+### Added
+- Initial copy of the v3 stubs and http client by [@vnephologist](https://github.com/vnephologist).
+
+### Changed
+- Change MessageResource.Details type from `map[string]string` and `map[string]interface{}` to `interface{}`
+