update CHANGELOG

Signed-off-by: Brian L. Troutwine <brian.troutwine@datadoghq.com>

Author: blt

CHANGELOG.md

@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.13.1-rc1]
+##
+- Introduce Datadog trace-agent payload support in JSON and MsgPack serialization.
+
 ## [0.13.0]
 ## Added
 - Introduced automatic throttling into generators to search for stable target load.