Add header for next set of changes

Author: omab

CHANGELOG.md

@@ -5,6 +5,8 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [Unreleased](https://github.com/python-social-auth/social-core/commits/master)
+
 ## [3.1.0](https://github.com/python-social-auth/social-core/releases/tag/3.1.0) - 2019-02-20
 
 ### Added