Add high-level architecture to README

README.md

@@ -0,0 +1,26 @@
+High-level architecture:
+
+```mermaid
+flowchart TD
+    Cellar[Cellar API]
+    Frontend
+
+    subgraph backend
+        client[Cellar Client]
+        conv[Formex -> HTML converter]
+        FastAPI
+    end
+
+    Cellar --> meta@{ shape: docs, label: "XML/RDF Metadata"}
+    Cellar --> docs@{ shape: docs, label: "Formex/XHTML/PDF text"}
+
+    meta --> client
+    docs --> client
+
+    client -- Formex --> FastAPI
+    client -- Formex --> conv
+
+    conv -- HTML --> FastAPI
+
+    FastAPI -- TOC, HTML article text --> Frontend
+```