Cleanup: upgrade.txt, README and CHANGES

[jrchamp]

From https://moodledev.io/docs/apis/commonfiles#changes

If your plugin includes a changelog in its root directory, this will be used to automatically pre-fill the release notes field when uploading new versions of your plugin to the Plugins directory.

Right now, our changelog is in README.me, but Moodle suggests having that text in CHANGES.md

From https://moodledev.io/docs/apis/commonfiles#upgradetxt

Each component and subsystem may make use of an upgrade.txt file in the top level folder. A section title is used to identify the Moodle version where the change was introduced, and significant changes for that version relating to that component or subsystem are noted.

Although I try to highlight these in the the release notes, noting them in the the upgrade.txt file is the Moodle way to mention significant changes.