Create a changelog.md file

[RobotSail]

When we issue new releases today, we typically just generate the release notes. But this doesn't paint the best picture when describing new features, improvements, and issues when we generate the release notes. And it's unrelealistic to expect the releaser to fully understand what each change enables, its implications, and how to best describe examples.

For this reason, we should adopt a changelog.md file as already exists in the CLI repo