Added CUAD dataset
1) Changed the language in a few places apart from those you mentioned in README
2) Reduced the size of dummy data folder by removing all other entries except the first
3) Updated YAML tags by using to the past version of `datasets-tagging` app. Will update the quick fix on that repository too in a while @bhavitvyamalik  Thanks for adding the dataset on huggingface! Can you please add a metric also for the dataset using the squad_v2 metric file?  @MohammedRakib you can check [#2257](https://github.com/huggingface/datasets/pull/2257)
Dataset link : https://github.com/TheAtticusProject/cuad/

Working on README.md currently.

Closes #2084 and [#1](https://github.com/TheAtticusProject/cuad/issues/1). 