Add CHANGELOG

Source:
https://github.com/reactphp/react/blob/a6de34d61f68adebd3cc3b855268a5f1475749b8/CHANGELOG.md

The following versions do not have a changelog at the moment:
v0.3.3, v0.3.2, v0.3.0, v0.2.6, v0.2.3, v0.2.0, v0.1.1