Add "How to earn a CNTi Certification badge" to README.md#cnti-certification-program

[lixuna]

What we have now on https://github.com/lfn-cnti/certification/blob/main/README.md#cnti-certification-program:

CNTi Certification Program

• Instructions

What we would like to see:

CNTi Certification Program

How to earn a CNTi Certification badge (BETA)

1. Prepare Learn about the self-certification requirements and technical prerequisites to prepare your product for certification.
2. Run the tests Follow the instructions to run your tests using the CNTi Test Suite.
3. Submit Prepare a PR to submit your results to the CNTi Certification repo on GitHub. Complete the participation form.
4. Review A reviewer will approve your PR at which point your product will be certified.