Introduce a change log markdown file

[j-c-cook]

I notice that you have a discussion of version 5 and its changes in your readme. (I appreciate your detailed overview of the repository in your readme.) My proposal is that you introduce a CHANGELOG.md file that contains the detailed changes from version to version. I am not proposing that you backlog the changelog, but rather begin documenting the future changes.

I have been using this technique, and will typically try to document changes from a branch in the CHANGELOG prior to merging a pull request in. I'll typically create a branch by the name issue#_ShortDescription and then merge into master once finalized. (See CONTRIBUTING.md.)

This is just an idea.