Create internationalization.md

This is a very basic documentation to getting started with internationalization with getx.

+## Translations
+Translations are kept as a simple key-value dictionary map.
+To add custom translations, create a class and extend `Translations`.
+```dart
+import 'package:get/get.dart';
+
+class Messages extends Translations {
+  @override
+  Map<String, Map<String, String>> get keys => {
+        'en_US': {
+          'hello': 'Hello World',
+        },
+        'de_DE': {
+          'hello': 'Hallo Welt',
+        }
+      };
+}
+```
+
+### Using translations
+Just append `.tr` to the specified key and it will be translated, using the current value of `Get.locale` and `Get.fallbackLocale`.
+```dart
+Text('title'.tr);
+```
+
+## Locales
+Pass parameters to `GetMaterialApp` to define the locale and translations.
+
+```dart
+return GetMaterialApp(
+    translations: Messages(), // your translations
+    locale: Locale('en_US'), // translations will be displayed in that locale
+    fallbackLocale: Locale('en_US'), // specify the fallback locale in case an invalid locale is selected.  
+    supportedLocales: <Locale>[Locale('en_US'), Locale('de_DE')] // specify the supported locales
+);
+```
+
+### Change locale
+Call `Get.updateLocale(locale)` to update the locale. Translations then automatically use the new locale.
+```dart
+var locale = Locale('en_US');
+Get.updateLocale(locale);
+```
+
+### System locale
+To read the system locale, you could use `Platform.localeName`.
+```dart
+return GetMaterialApp(
+    locale: Locale(Platform.localeName),
+);
+```