Add CHANGELOG; fixes #465 (#522)

CHANGELOG.md

+# Changelog
+
+All changes that impact users of 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).
+
+<!---
+This document is intended for users of the applications and API. Changes to things
+like tests should not be noted in this document.
+
+When updating this file for a PR, add an entry for your change under Unreleased
+and one of the following headings:
+ - Added - for new features.
+ - Changed - for changes in existing functionality.
+ - Deprecated - for soon-to-be removed features.
+ - Removed - for now removed features.
+ - Fixed - for any bug fixes.
+ - Security - in case of vulnerabilities.
+
+If the heading does not yet exist under Unreleased, then add it as a 3rd heading,
+with three #.
+
+
+When preparing for a public release candidate add a new 2nd heading, with two #, under
+Unreleased with the version number and the release date, in year-month-day
+format. Then, add a link for the new version at the bottom of this document and
+update the Unreleased link so that it compares against the latest release tag.
+
+
+When preparing for a bug fix release create a new 2nd heading above the Fixed
+heading to indicate that only the bug fixes and security fixes are in the bug fix
+release.
+-->
+
+## [Unreleased]
+