Adding SICK dataset
Hi
It would be great to include SICK dataset.

## Adding a Dataset
- **Name:** SICK
- **Description:** a well known entailment dataset 
- **Paper:** http://marcobaroni.org/composes/sick.html
- **Data:** http://marcobaroni.org/composes/sick.html
- **Motivation:** this is an important NLI benchmark

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



thanks