docs: update CHANGELOG.md format

This aligns it with the suggested format at https://keepachangelog.com/en/1.0.0/.

Author: arxanas

CHANGELOG.md

@@ -5,18 +5,18 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## Unreleased
+## [Unreleased]
 
 - BREAKING: Events are now grouped into transactions. This improves the UX around `git undo`, since it can undo groups of related events. This breaks the on-disk database format.
 
-## v0.2.0 - 2020-03-15
+## [0.2.0] - 2020-03-15
 
 Ported to Rust. No new features.
 
 - Performance for repeated calls to Git hooks is significantly improved. This can happen when rebasing large commit stacks.
 - The `git undo` UI has been changed to use a Rust-specific TUI library (`cursive`).
 
-## v0.1.0 - 2020-12-18
+## [0.1.0] - 2020-12-18
 
 First beta release. Supports these commands: