Update/tested up to 5.6

[jeffpaul]

Description of the Change

David Chabbi helped test and validate that things work as expected on WP 5.6, so this PR bumps the "tested up to" value to 5.6 and credits David for his testing.

Alternate Designs

n/a

Benefits

Ensures we properly state our tested-up-to version on GitHub and WP.org

Possible Drawbacks

none identified

Verification Process

David tested the plugin locally.

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.

Applicable Issues

Closes #8.

Changelog Entry