docs: point API and README at the price-tag design

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

recognization/docs/API.md

@@ -8,6 +8,8 @@ once; no client shares state with another.
 - Web client (served by the server itself): `http://127.0.0.1:8080/`
 - Reference non-browser client: `code/stream_client.py`
 - Interactive API docs: `http://127.0.0.1:8080/docs`
+- Planned extensions (price-tag OCR, reconciliation, compliance, mobile scan):
+  `docs/design-price-tags-and-compliance.md`
 
 ## Running it