Add a CHANGELOG (#57)

This adds a CHANGELOG.md to the project, retroactively filling it up
based on https://github.com/uber-go/atomic/releases.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,6 @@
 Before opening your pull request, please make sure that you've:
 
+- [ ] updated the changelog if the change is user-facing;
 - [ ] [signed Uber's Contributor License Agreement](https://docs.google.com/a/uber.com/forms/d/1pAwS_-dA1KhPlfxzYLBqK6rsSWwRwH95OCCZrcsY5rk/viewform);
 - [ ] added tests to cover your changes;
 - [ ] run the test suite locally (`make test`); and finally,

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).
+
+## [1.4.0] - 2019-05-01
+### Added
+ - Add `atomic.Error` type for atomic operations on `error` values.
+
+## [1.3.2] - 2018-05-02
+### Added
+- Add `atomic.Duration` type for atomic operations on `time.Duration` values.
+
+## [1.3.1] - 2017-11-14
+### Fixed
+- Revert optimization for `atomic.String.Store("")` which caused data races.
+
+## [1.3.0] - 2017-11-13
+### Added
+- Add `atomic.Bool.CAS` for compare-and-swap semantics on bools.
+
+### Changed
+- Optimize `atomic.String.Store("")` by avoiding an allocation.
+
+## [1.2.0] - 2017-04-12
+### Added
+- Shadow `atomic.Value` from `sync/atomic`.
+
+## [1.1.0] - 2017-03-10
+### Added
+- Add atomic `Float64` type.
+
+### Changed
+- Support new `go.uber.org/atomic` import path.
+
+## [1.0.0] - 2016-07-18
+
+- Initial release.
+
+[1.4.0]: https://github.com/uber-go/atomic/compare/v1.3.2...v1.4.0
+[1.3.2]: https://github.com/uber-go/atomic/compare/v1.3.1...v1.3.2
+[1.3.1]: https://github.com/uber-go/atomic/compare/v1.3.0...v1.3.1
+[1.3.0]: https://github.com/uber-go/atomic/compare/v1.2.0...v1.3.0
+[1.2.0]: https://github.com/uber-go/atomic/compare/v1.1.0...v1.2.0
+[1.1.0]: https://github.com/uber-go/atomic/compare/v1.0.0...v1.1.0
+[1.0.0]: https://github.com/uber-go/atomic/releases/tag/v1.0.0