Add Changelog Entry

Add entries to the Simple History plugin's readme.txt changelog.

Workflow

1. Ask user for the change description (if not provided)
2. Determine category: Added, Changed, Fixed, Deprecated, Removed, Security
3. Add entry under ## Changelog → ### Unreleased
4. Confirm with user

Format

-   Fixed post creation via Gutenberg autosave not being logged. [#599](https://github.com/bonny/WordPress-Simple-History/issues/599)