ignored_files: example suggestion

Hi,

You'd do a favor to a lot of people who deal with regular expressions just once in a long while. Add to README a couple of variants, something like this:
```
import_sorter:
  ignored_files:
    - \/.*\.gr\.dart
    - \/.*\.freezed\.dart
```
Looks like a trifle, but really it's up to fifteen minutes' waste of time per [not so bright] a person like me...