ignored_files: example suggestion

[Tyrn]

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...

[patachi]

- \/lib\/* : OK

- \/lib\/*.pb.dart : Fail
- \/lib\/protos\/* : Fail
- \/lib\protos\* : Fail
- \/lib/protos/* : Fail
- \lib\protos\* : Fail
- /lib/protos/* : Fail