Add changelog file (CHANGELOG.md)

Problem/Motivation

Keeping a changelog is a good best practice to follow.

Proposed resolution

Use Keep a Changelog.

Because we are using mkdocs, we could probably put this file in the root of the repository and symlink it from /docs, the same way README.md is. This would allow it work with both GitLab and mkdocs.

Remaining tasks

• Add the file to the repository root.
• Symlink to it from the docs directory.

User interface changes

This will add a new page to the mkdocs documentation.

• A new page will be added to the mkdocs documentation.
• GitLab should display a CHANGELOG link on the repository homepage.

API changes

N/A

Data model changes

N/A