Update the change log

CHANGELOG.md

@@ -13,6 +13,12 @@ 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]
+
+### Fixed
+- A race condition when requesting multiple certificates on the same non-existent account has been fixed.
+
+
 ## [0.6.0] - 2019-06-05
 
 ### Added