Update related README

Author: minhdanh

flutter_cache_manager_firebase/README.md

@@ -6,9 +6,16 @@ A Firebase implementation for [flutter_cache_manager](https://pub.dev/packages/f
 
 This library contains FirebaseCacheManager and FirebaseHttpFileService.
 
-You can easily fetch a file stored on Firebase with 
+You can easily fetch a file stored on Firebase with
 ```dart
 var file = await FirebaseCacheManager().getSingleFile(url);
 ```
 
-You could also write your own CacheManager which uses the FirebaseHttpFileService.
+You could also write your own CacheManager which uses the FirebaseHttpFileService.
+
+### Custom Firebase storage bucket
+
+You can use a custom bucket by passing `bucket` parameter. For example:
+```dart
+FirebaseCacheManager(bucket: "my-bucket");
+```