Update README.md

Author: nicolasgarnier

README.md

@@ -92,6 +92,10 @@ firebase.initializeApp(config);
 
 class SignInScreen extends React.Component {
 
+  state = {
+    signedIn: false // Local signed-in state.
+  };
+  
   // Configure FirebaseUI.
   uiConfig = {
     // Popup signin flow rather than redirect flow.
@@ -105,15 +109,11 @@ class SignInScreen extends React.Component {
     callbacks: {
       signInSuccess: () => {
         this.setState({signedIn: true});
-        return false;
+        return false; // Avoid redirects after sign-in.
       }
     }
   };
 
-  state = {
-    signedIn: false
-  };
-  
   render() {
     if (!this.state.signedIn) {
       return (