Link the app usage guide

docs/README.md

@@ -62,3 +62,7 @@ Browser → public/index.php → Request → Dispatcher → Router → Route →
 ## Flow chart
 
 See [`REQUEST_FLOW.md`](./REQUEST_FLOW.md) for a chart of how requests and responses move through the framework.
+
+## Usage guide
+
+See [`USAGE.md`](./USAGE.md) for a user guide that explains the app’s main features and workflows.