Consider renaming CHANGELOG to CHANGELOG.md

[heynan0]

Consider turning the plain text CHANGELOG into a markdown file.

That way, we can leverage linking to the pull requests that added new features or made other kinds of changes. Writing #123 will turn into a link where others can quickly reference when they are reading it and need to upgrade to a newer version. They will have confidence about the changes that are introduced and can check it won't break their application.

[brandur]

This project was started long enough ago that the Markdown changelog wasn't a super common standard quite yet so I think I just left things as plaintext. I'm not against going to Markdown though — we'd just want to make sure to update references elsewhere to it and maybe leave a CHANGELOG file in place with a hint that changes are now located in CHANGELOG.md.