Add test & mock example

example/src/__mocks__/react-native-i18n.js

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+// @flow
+
+import I18nJs from 'i18n-js';
+
+I18nJs.locale = 'en'; // a locale from your available translations
+export const getLanguages = () => Promise.resolve('en');
+export default I18nJs;

example/src/__tests__/App.js

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+import 'react-native';
+import React from 'react';
+import App from '../App.js';
+
+// Note: test renderer must be required after react-native.
+import renderer from 'react-test-renderer';
+
+it('renders correctly', () => {
+  const tree = renderer.create(
+    <App />
+  );
+});