Completion handler snippet

## 🏗 Enhancement Proposal

Include small snippet to usage - might help Flutter newbies like myself :) Had to spend quite a plenty of time to wrap my head around what's going on in the code to implement.
```dart
CachedNetworkImageProvider cacheProviderWithEvent(String url, Function onLoadComplete) {
var manager = DefaultCacheManager();
manager.getFileStream(url).listen((event) {
if (event !is FileInfo) {
onLoadComplete?.call();
}
});
return CachedNetworkImageProvider(url, cacheManager: manager);
}
```

### Pitch

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android