Add bibtex reference to README

[nmaterise]

To cite our work, use this bibtex

@misc{scresonators,
  title        = {{boulder-cryogenic-quantum-testbed/scresonators}},
  year         = 2023,
  howpublished = {\url{https://github.com/Boulder-Cryogenic-Quantum-Testbed/scresonators}}
}