Add CHANGELOG.md

[BrianSipple]

Right now, the Marketplace page for this project contains a wonderful Changelog:

However, I think it would be even more helpful to start maintaining that here as a CHANGELOG.md file, as per the extension recommendations after 1.7. That way, it can also show up for anyone viewing the project details page within their editor:

[sean-mcmanus]

Yes, we are aware of this. It has been on our TODO list.

[bobbrow]

This issue has been fixed with v0.11.0.