add firebase react native code example

Example 1: react native firebase

// Using npm
npm install --save @react-native-firebase/app

# create your firebase android project and follow the steps for implementations. 
# check: https://rnfirebase.io/#generating-android-credentials

Example 2: react native firebase

/* Answer to: "react native firebase" */

/*
  React Native Firebase is the officially recommended collection
  of packages that brings React Native support for all Firebase
  services on both Android and iOS apps.
*/