Prepare release notes for 5.0.0-dev13 (#6023)

Author: eddyashton

CHANGELOG.md

@@ -5,7 +5,9 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## Unreleased
+## [5.0.0-dev13]
+
+[5.0.0-dev13]: https://github.com/microsoft/CCF/releases/tag/ccf-5.0.0-dev13
 
 ### Fixed