docs: Update README.md with open api spec

[KClough]

Summary:

Resolves #1417

Expected behavior:

Explain and/or show screenshots for how you expect the pull request to work in your testing (in case other devices exhibit different behavior).

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[davidgamez]

Nice addition!