Add CHANGELOG.md

Add a CHANGELOG.md with information including changes in the currently
unreleased (soon to be 0.2.0) version.

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]
+### Added
+- `SetEmission` API
+- `GetMissionStartTime` API
+- `GetScenarioCurrentTime` API
+- `GetBullseye` API
+- `GetTransform` API
+- `AddGroup` API (Initial version suitable for spawning fixed SAM sites)
+- `Eval` API executable in the DCS hook environment along with command-line tool
+- Ability to include DCS-gRPC on all server missions without needing to edit the mission
+- Updated Rust version and many Rust dependencies
+
+### Changed
+- Split and reorganised APIs into versioned namespaces
+- Switched to a different way of initialising the server that does not require sanitisation
+- Changed Enum numbering to allow more idiomatic gRPC usage
+
+## [0.1.0] - 2021-1-23
+### Added
+- Initial release with APIs documented in https://github.com/DCS-gRPC/rust-server/wiki/API-documentation-0.1.0