docs: Added Expo's appearance suggestion (#586)

Author: jasuno

README.md

@@ -36,6 +36,18 @@ If your project is using [Expo](https://expo.io/), install the library and the c
 expo install react-native-modal-datetime-picker @react-native-community/datetimepicker
 ```
 
+To ensure the picker theme respects the device theme, you should also configure the appearance styles in your `app.json` this way:
+
+```json
+{
+  "expo": {
+    "userInterfaceStyle": "automatic"
+  }
+}
+```
+
+Refer to the [Appearance](https://docs.expo.io/versions/latest/sdk/appearance/) documentation on [Expo](https://expo.io/) for more info.  
+
 ## Usage
 
 ```javascript