Write a changelog to be presented by Package Control

https://packagecontrol.io/docs/messaging

Use this as an opportunity to solicit feedback from Linux users regarding whether the [current Linux keyboard shortcuts](https://github.com/frou/st3-gitblame/blob/master/Keymaps/Default%20(Linux).sublime-keymap) are appropriate, since I have no idea whether they conflict with global keyboard shortcuts already used by GNOME/KDE/etc.