docs: add translation.md

Author: LucasXu0

frontend/app_flowy/packages/appflowy_editor/documentation/translation.md

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+# Translate AppFlowy Editor
+
+You can help Appflowy Editor in supporting various languages by contributing. Follow the steps below sequentially to contribute translations.
+
+## Steps to modify an existing translation
+Translation files are located in: `frontend/app_flowy/packages/appflowy_editor/lib/l10n/`
+1. Install the Visual Studio Code plugin: [Flutter intl](https://marketplace.visualstudio.com/items?itemName=localizely.flutter-intl)
+2. Modify the specific translation file.
+3. Save the file and the translation will be generated automatically.
+
+## Steps to add new language
+Translation files are located in: `frontend/app_flowy/packages/appflowy_editor/lib/l10n/`
+1. Install the Visual Studio Code plugin: [Flutter intl](https://marketplace.visualstudio.com/items?itemName=localizely.flutter-intl)
+2. Copy the `intl_en.arb` as a base translation and rename the new file to `intl_<new_locale>.arb`
+3. Modify the new translation file.
+4. Save the file and the translation will be generated automatically.