adding ccnet dataset
closing since I think this is cc100, just the name has been changed. thanks 
## Adding a Dataset
- **Name:**  ccnet

- **Description:** 
Common Crawl

- **Paper:** 
https://arxiv.org/abs/1911.00359

- **Data:** 
https://github.com/facebookresearch/cc_net

- **Motivation:**
this is one of the most comprehensive clean monolingual datasets across a variety of languages. Quite important for cross-lingual reseach

Instructions to add a new dataset can be found [here](https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md).

thanks