Rename categories (Changed, Added, Removed, Fixed)

The current categories (Changed, Added, Removed, Fixed) are from Keep a Changelog. It's awkward that the categories use past tense while changes use present tense. E.g.:

```md
### Added

- Add minimize button
```

An alternative could be Changes, Additions, Removals, Fixes. E.g.:

```md
### Additions

- Add minimize button
```

Thoughts welcome.