Create CHANGELOG

Add a change log to the project following the guidelines set by Keep a Changelog: https://keepachangelog.com/en/1.0.0/

Author: jeancochrane

CHANGELOG.md

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+# Change Log
+
+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).
+
+## [1.0.1](https://github.com/jeancochrane/pytest-flask-sqlalchemy/releases/tag/v1.0.1) (2019-03-02)
+
+### Changed
+
+- Expire session state at the top level [\#6](https://github.com/jeancochrane/pytest-flask-sqlalchemy/pull/6)