basic Create React App working

frontend/src/app/reducers.js

@@ -0,0 +1,13 @@
+
+// from https://github.com/supasate/connected-react-router
+import { combineReducers } from 'redux'
+import { connectRouter } from 'connected-react-router'
+import counterReducer from '../features/counter/counterSlice';
+
+const createRootReducer = (history) => combineReducers({
+  router: connectRouter(history),
+  counter: counterReducer,
+})
+export default createRootReducer
+
+