bump WP tested-up-to version 5.9

[jeffpaul]

Co-Authored-By: kojraai 27773149+kojraai@users.noreply.github.com

Description of the Change

This adds a WP tested-up-to badge in readme.md as there's no readme.txt file. If we were to publish this to WP.org, then we could look to add a readme.txt file and note the tested-up-to version there.

Closes #63.

Alternate Designs

n/a

Possible Drawbacks

none identified

Verification Process

Manually verified via VS Code markdown viewer.

Checklist:

• [x] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests passed.

Changelog Entry

Changed - Bumped WordPress "tested up to" version 5.9.

Credits

Props @kojraai