added spans field for the wikiann datasets
Hi @lhoestq 
Thanks a lot for taking time checking it. I update "dataset_infos.json", I added description to the function of _generate_samples  in wikiann.py but I was not sure about the format to write in README. thanks.  Thanks !

For the fields description in the dataset card, something like this does the job:
```
- `tokens`: a `list` of `string` features.
- `langs`: a `list` of `string` features that correspond to the language of each token.
- `ner_tags`: a `list` of classification labels, with possible values including `O` (0), `B-PER` (1), `I-PER` (2), `B-ORG` (3), `I-ORG` (4), `B-LOC` (5), `I-LOC` (6).
- `spans`: a `list` of `string` features, that is the list of named entities in the input text formatted as ``<TAG>: <mention>``
```

Also for information, I think the trailer of rick and morty season 5 is out now :) Hi @lhoestq 
thank you! This is updated now, please feel free to let me know if I need to modify something :)  thanks 
Hi @lhoestq 
I tried to add spans to the wikiann datasets.
Thanks a lot for kindly having a look.
This addresses https://github.com/huggingface/datasets/issues/2130. 
Best regards
Rabeeh 