Document bridge spec

[pladaria]

The original documentation is deprecated:
https://telefonicacorp.sharepoint.com/:w:/s/Colabora_TCX/EeE60O1XszBPgShpuKjRZ90BnklpXTiwFgKTtBTQcEfSOw?e=e3QRMw

Create a new document in this repo documenting:

• Protocol specification
• Error handling