chore: Add changelog for rollouts beta. (#74)

Author: Michael Ng

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](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+Changes that have landed but are not yet released.
+- Support for running full feature tests with the SDK.
+
+## [0.1.0-beta] - August 23rd, 2019
+This is the initial release of the SDK, which includes support for running Feature Rollouts using Optimizely Feature Management.
+
+### New Features
+- Introduces the `IsFeatureEnabled` API for determining whether the feature is enabled for a user or not. Please see our [README](README.MD).