Update CHANGELOG.md

Author: andystaples

CHANGELOG.md

@@ -5,6 +5,12 @@ 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
+
+ADDED
+
+- Added `durabletask.testing` module with `InMemoryOrchestrationBackend` for testing orchestrations without a sidecar process
+
 ## v1.3.0
 
 ADDED