docs: add ARCHITECTURE.md from Palace doc, cross-reference from README

README.md

@@ -39,6 +39,11 @@ Both valves are passed at runtime — no hardcoded keys. The tool is portable to
 - **General-purpose.** Works with any LiteLLM proxy instance.
 - **Read-only.** GET endpoints only. No mutations, no risk.
 
+## Documentation
+
+- **[ARCHITECTURE.md](ARCHITECTURE.md)** — Full architecture and design doc: API endpoints, response processing, stripped fields, error paths, future considerations.
+- **This README** — Quick reference: install, functions, valves, design principles.
+
 ## License
 
 MIT