Add changelog. (#72)

* Add changelog.

* Add description.

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
+- Update Kotlin to 1.3.61
+- Update Kotlin Coroutines to 1.3.3
+
+### Removed
+- Samples moved to https://github.com/oolong-kt/oolong-samples
+
+## [2.0.1] - 2019-06-20
+### Added
+- Update Kotlin to 1.3.40
+
+### Changed
+- Remove default arguments for Runtime constructor
+
+## [2.0.0] - 2019-05-31
+### Added
+- Add samples
+
+### Changed
+- Move to oolong-kt org and update packages
+- Change core types from classes to functions
+
+## [1.0.0] - 2018-08-01
+
+Initial release
+
+[Unreleased]: https://github.com/oolong-kt/oolong/compare/v2.0.1...HEAD
+[2.0.1]: https://github.com/oolong-kt/oolong/compare/v2.0.0...v2.0.1
+[2.0.0]: https://github.com/oolong-kt/oolong/compare/v1.0.0...v2.0.0
+[1.0.0]: https://github.com/oolong-kt/oolong/releases/tag/v1.0.0
+