Add CHANGELOG.md

Author: justinfagnani

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/).
+
+<!--
+   PRs should document their user-visible changes (if any) in the
+   Unreleased section, uncommenting the header as necessary.
+-->
+
+## Unreleased
+
+### Added
+* Added `@query()`, `@queryAll()`, and `@customElement` decorators ([#159](https://github.com/Polymer/lit-element/pull/159))
+
+<!-- ### Changed -->
+<!-- ### Removed -->
+<!-- ### Fixed -->