Add CHANGELOG

Source:
https://github.com/reactphp/react/blob/a6de34d61f68adebd3cc3b855268a5f1475749b8/CHANGELOG.md

CHANGELOG.md

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+# Changelog
+
+## 0.4.0 (2014-02-02)
+
+* BC break: Drop unused `Response::getBody()`
+* BC break: Bump minimum PHP version to PHP 5.4, remove 5.3 specific hacks
+* BC break: Remove `$loop` argument from `HttpClient`: `Client`, `Request`, `Response`
+* BC break: Update to React/Promise 2.0
+* Dependency: Autoloading and filesystem structure now PSR-4 instead of PSR-0
+
+## 0.3.1 (2013-04-21)
+
+* Bug fix: Correct requirement for socket-client
+
+## 0.3.0 (2013-04-14)
+
+* BC break: Socket connection handling moved to new SocketClient component
+
+## 0.2.6 (2012-12-26)
+
+??
+
+## 0.2.5 (2012-11-26)
+
+* Feature: Use a promise-based API internally
+* Bug fix: Use DNS resolver correctly
+
+## 0.2.3 (2012-11-14)
+
+??
+
+## 0.2.2 (2012-10-28)
+
+* Feature: HTTP client (@arnaud-lb)