Add OpenSSF best practices badge

[felddy]

🗣 Description

This PR adds the Open Source Security Foundation (OpenSSF) best practices badge.

See:

• https://bestpractices.coreinfrastructure.org/en
• https://bestpractices.coreinfrastructure.org/en/projects

💭 Motivation and Context

The lack of this badge was a finding of the recently added code scanning tool scorecard.

See:

• https://github.com/ossf/scorecard#readme

🧪 Testing

Tested locally and with CI suite.

✅ Checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow project standards.
• All relevant repo and/or project documentation has been updated to reflect
  the changes in this PR.
• All new and existing tests pass.