Introduce CHANGELOG.md (#1153)

Co-authored-by: Steven L <imgroxx@gmail.com>

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]
+### Added
+- Added JWT Authorization Provider. This change includes a dependency that uses v2+ go modules. They no longer match import paths, meaning that we have to **drop support for dep & glide** in order to use this. [#1116](https://github.com/uber-go/cadence-client/pull/1116)
+### Changed
+- Generated proto type were moved out to [cadence-idl](https://github.com/uber/cadence-idl) repository. This is **BREAKING** if you were using `compatibility` package. In that case you will need to update import path from `go.uber.org/cadence/.gen/proto/api/v1` to `github.com/uber/cadence-idl/go/proto/api/v1` [#1138](https://github.com/uber-go/cadence-client/pull/1138)
+### Documentation
+- Documentation improvements for `client.SignalWorkflow` [#1151](https://github.com/uber-go/cadence-client/pull/1151)
+
+
+## [v0.18.5] - 2021-11-09