Update navigation

Author: flexbox

challenges/react-navigation/04.md

@@ -4,12 +4,26 @@
 
 - Add an auth flow
 
-## 👨‍🚀 Exercise 4 Authentication
+## 👨‍🚀 Exercise 4
 
 - [ ] Add an Authentication flow to your application
 
+Example:
+
+```javascript
+function App() {
+  const [isSignedIn, setIsSignedIn] = useState(false);
+
+  return (
+    <NavigationContainer>
+      {isSignedIn ? <RootNavigator /> : <AuthNavigator />}
+    </NavigationContainer>
+  );
+}
+```
+
 You can read the documentation [https://reactnavigation.org/docs/auth-flow](https://reactnavigation.org/docs/auth-flow)
-_Hint:_ For now you can emulate the token with a simple `useState`. Later we will see how to use `AsyncStorage` to keep the token on the phone.
+_Hint:_ For now you can emulate the token with a simple `useState`.
 
 ### 👽 Bonus