feat: add sample multi-user support

feat: update README

client/src/store/userReducer.js

@@ -0,0 +1,19 @@
+import { createSlice } from '@reduxjs/toolkit';
+
+const initialState = {
+  user: null,
+};
+
+const currentSlice = createSlice({
+  name: 'user',
+  initialState,
+  reducers: {
+    setUser: (state, action) => {
+      state.user = action.payload;
+    },
+  }
+});
+
+export const { setUser } = currentSlice.actions;
+
+export default currentSlice.reducer;