add WP and PHP minimum badges

[jeffpaul]

Description of the Change

This PR adds badges to note the WordPress and PHP minimums for this plugin. I did NOT remove the "Requirements" section of the README.md file, but I wonder if that might be worthwhile so that we don't need to worry about remembering to update those references in the future (as the badges auto-update based on what's shown on the WP.org repo)?

How to test the Change

Use GitHub markdown previewer to see updated badges.

Changelog Entry

n/a

Credits

Props @jeffpaul.

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly.
• [ ] I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• [ ] All new and existing tests pass.