Updated changelog

CHANGELOG.md

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+# Changelog
+
+All Notable changes to `gravitymedia/urn` will be documented in this file.
+This project adheres to [Semantic Versioning](http://semver.org/).
+
+## NEXT - YYYY-MM-DD
+
+### Added
+- Nothing
+
+### Changed 
+- Nothing
+
+### Deprecated
+- Nothing
+
+### Removed
+- Nothing
+
+### Fixed
+- Nothing
+
+### Security
+- Nothing
+
+## v0.2.0 - 2016-06-30
+
+### Added
+- An `UrnInterface` with necessary methods for the URN object
+
+### Changed
+- The `Urn` object is now immutable
+
+## v0.1.0 - 2015-02-20
+
+### Added
+- Initial implementation of an `Urn` object to represent an URN