add missing `Support Level` section

[jeffpaul]

Summary

This PR adds in the missing Support Level section to the README.md file so that the Support Level badge link works correctly (e.g., has something to link TO).

Contributor checklist

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly
• [x] I have added tests to cover changes introduced by this pull request
• [x] All new and existing tests pass

Testing

n/a

Reviewer checklist

- [x] Code changes review - [x] Documentation changes review - [x] Unit tests pass - [ ] Static analysis passes

[coveralls]

coverage: 59.6% (-0.1%) from 59.733% when pulling 3fd8ebe38e5c107adbe7e015e4c4a684302afd59 on add/support-level into 1937bad516909d4e6155ab2a8bde4816e91eeee5 on trunk.