docs(misc): add note to enable firebase auth provider (bizz84#61)

this will prevent a CONFIGURATION_NOT_FOUND exception when attempting
any sign-in methods

Author: Amir Tahvildaran

README.md

@@ -349,6 +349,7 @@ To use this project with Firebase, some configuration steps are required.
 - then, [download and copy](https://firebase.google.com/docs/flutter/setup#configure_an_android_app) `google-services.json` into `android/app`.
 - On iOS, use `com.example.starterArchitectureFlutterFirebase` as the bundle ID.
 - then, [download and copy](https://firebase.google.com/docs/flutter/setup#configure_an_ios_app) `GoogleService-Info.plist` into `iOS/Runner`, and add it to the Runner target in Xcode.
+- finally, enable the Email/Password Authentication Sign-in provider in the Firebase Console (Authentication > Sign-in method > Email/Password > Edit > Enable > Save)
 
 See this page for full instructions:
 
@@ -419,4 +420,4 @@ Other relevant articles about app architecture:
 - [Widget-Async-Bloc-Service: A Practical Architecture for Flutter Apps](https://codewithandrea.com/articles/2019-05-21-wabs-practical-architecture-flutter-apps/)
 - [Flutter TDD Clean Architecture Course [1] – Explanation & Project Structure](https://resocoder.com/2019/08/27/flutter-tdd-clean-architecture-course-1-explanation-project-structure/)
 
-## [License: MIT](LICENSE.md)
+## [License: MIT](LICENSE.md)