Add changelog

Signed-off-by: Mike Seplowitz <[email protected]>

Author: mikesep

CHANGELOG.markdown

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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 ([diff][unreleased-diff])
+
+## [0.1.0][] - 2019-05-24
+
+First release of the library.
+
+[unreleased-diff]:
+  https://github.com/bloomberg/go-testgroup/compare/v0.1.0...HEAD
+[0.1.0]: https://github.com/bloomberg/go-testgroup/releases/tag/v0.1.0