Add translation guide doc

doc/TRANSLATION.md

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+# TRANSLATION
+
+You can help Appflowy in supporting various languages by contributing. Follow the steps below sequentially to contribute translations.
+
+**NOTE: Translation files SHOULD be** `json` **files named in the format** `<lang_code>-<country_code>.json` **or just** `<lang_code>.json`**. eg:**`en.json`**,** `en-UK.json`
+
+## Steps to add new language support
+
+1. Add language key-value json file to `frontend/app_flowy/assets/translations/`. Refer `en.json` for format and keys.
+2. Run `flutter pub run easy_localization:generate -S assets/translations/`.
+3. Run `flutter pub run easy_localization:generate -f keys -o locale_keys.g.dart -S assets/translations`.
+4. Add locale of the language (eg: `Locale('en', 'IN')`, `Locale('en')`) in `supportedLocales` list under `EasyLocalization` wrapper for flutter to support it.
+This is located in `frontend/app_flowy/lib/startup/tasks/application_widget.dart` under `AppWidgetTask` class as shown below:
+
+    ```dart
+    runApp(
+      EasyLocalization(
+          supportedLocales: const [ Locale('en') ], // <---- Add locale to this list
+          path: 'assets/translations',
+          fallbackLocale: const Locale('en'),
+          child: app),
+    );
+    ```
+
+## Steps to modify translations
+
+1. Modify the specific translation file.
+2. Run `flutter pub run easy_localization:generate -S assets/translations/`.
+3. Run `flutter pub run easy_localization:generate -f keys -o locale_keys.g.dart -S assets/translations`.