Document backend client configuration

README.md

@@ -27,6 +27,10 @@ backend. The URL usually has this format:
 https://hub.example.com/api/client/config/global
 ```
 
+Enable the `client-config` external plugin in the backend configuration. The
+backend `abbr` value must match the API prefix without its leading slash. For
+example, use `"abbr": "api"` with `"prefix": "/api"`.
+
 The frontend uses the Vite values during the image build. Rebuild the image after
 you change `.env`.