Add CHANGELOG.md

Author: tessereth

CHANGELOG.md

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+# Change Log
+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
+
+### Removed
+
+### Changed
+
+## [v0.4.6](https://github.com/bugcrowd/vrt-ruby/compare/v0.4.5...v0.4.6) - 2018-02-05
+### Changed
+- Cache VRT::Map objects (#18)