Please update the changelog for cherry_picker

[Mariatta]

@abadger would you mind updating the changelog to include changes made in your PRs #264, #265, #266?

Thanks.

[Mariatta]

GitMate.io thinks possibly related issues are #227 (Initial Update), #114 (publish cherry_picker on PyPI ?), #140 (Move cherry_picker over to flit), #50 (Add a setup command to cherry_picker), and #58 (Set labels in cherry_picker.py).