OXDEV-479 Add changelog

Author: sebbbbauer

CHANGELOG.md

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+# Change Log for OXID eShop testing library
+
+All notable changes to this project will be documented in this file.
+The format is based on [Keep a Changelog](http://keepachangelog.com/)
+and this project adheres to [Semantic Versioning](http://semver.org/).
+
+## [Unreleased]
+
+### Added
+
+### Changed
+
+### Deprecated
+
+### Removed
+
+### Fixed
+
+### Security
+
+## [3.3.0] - 2017-12-05
+
+### Added
+
+- add helping methods markTestSkippedIfSubShop and markTestSkippedIfNoSubShopto the library/UnitTestCase