Create changelog for 2.x

Author: barryvdh

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](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [2.0.0] - 2020-05-11
+
+### Added
+- CORS headers are better cachable now, with correct Vary headers (#70, #74)
+
+### Changed
+- CORS headers are added to non-Origin requests when possible (#73)
+- Requests are no longer blocked by the server, only by the browser (#70)