[docs][core] default app additions

docs/core/config-default-app.md

@@ -1,4 +1,16 @@
-<!-- TODO -->
+### Default Firebase App
+
+After following the iOS & Android install guides and correctly setting up your google services plist/json files; the default app is automatically initialized and available for use in react-native-firebase.
+
+There's no need to call `initializeApp(opt)` in your JS code for the default app, import RNFirebase and use the default app straight away:
+
+```javascript
+import firebase from 'react-native-firebase';
+
+console.log(firebase.database().app.name); // '[DEFAULT]'
+```
+
+!> Calling `initializeApp()` for the default app will throw an 'app already initialized' error in a later release.
 
 ### Enable Database Persistence