Add a diagram showing how identity-registry fits between backend and Stellar network

## Description
A single picture of the whole system (WhatsApp -> backend -> identity-registry -> Stellar) would orient new contributors faster than reading three READMEs across repos.

## Deliverables
- A mermaid diagram in README.md showing the high-level flow: WhatsApp message -> backend -> identity-registry contract -> Stellar ledger, with a one-line caption per arrow.

## Depends on
None
