Add CHANGELOG.md

Summary: Add CHANGELOG.md

Differential Revision: D17282737

fbshipit-source-id: a05585096ba0c3c506853f6e17c9d815ff6defd8

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]
+
+### Fixed
+
+- Fix Android logout issue
+- Fix the issue of multiple AppRequest instances when changing screen orientation
+
+### Changed
+
+- Remove Bolts framework
+
+## [7.17.2] - 2019-07-18
+
+### Fixed
+
+- Various bug fixes
+
+## [7.17.1] - 2019-07-05
+
+### Other
+
+- Facebook Developer Docs: [Changelog v7.x](https://developers.facebook.com/docs/unity/change-log)
+
+<!-- Links -->
+
+[Unreleased]: https://github.com/facebook/facebook-sdk-for-unity/compare/sdk-version-7.17.2...HEAD
+[7.17.2]: https://github.com/facebook/facebook-sdk-for-unity/compare/sdk-version-7.17.1...sdk-version-7.17.2
+[7.17.1]: https://github.com/facebook/facebook-sdk-for-unity/compare/sdk-version-7.1.0...sdk-version-7.17.1