Would be awesome to have a Widget Testing recipe in the readme!

## 💬 Questions and Help

I was trying to follow on old [medium article](https://medium.com/flutter-community/golden-testing-using-cachednetworkimage-1b488c653af3) linked in a [stack overflow question](https://stackoverflow.com/questions/59408845/how-to-write-widget-test-for-cachednetworkimage-in-flutter) to figure out how to write a widget test for a component that relies on CacheManager.

Unfortunately, it seems like a new version significantly changed the API and the recipe provided there is all kinds of broken. I wonder if it would be possible for someone to link from the readme a recipe for how to write such a test (possibly by implementing https://github.com/Baseflow/flutter_cache_manager/blob/develop/flutter_cache_manager/example/test/widget_test.dart and linking to it?)

flutter_cache_manager/example/test/widget_test.dart から始め、issue に記載されている古いレシピと比較してください。README を確認して、widget testing のレシピをどこにリンクすべきかを特定してください。完了とは、例が現在の API で動作し、README が読者をそのレシピに案内している状態です。

索引モデルが issue の本文から書いたものです。