Add missing "Unreleased" section to CHANGELOG
@@ -5,6 +5,8 @@ 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
+
## [v2.0.0](https://github.com/allenai/allennlp/releases/tag/v2.0.0) - 2021-01-27
