Add "Adversarial GLUE" dataset to datasets library
_The documentation is not available anymore as the PR was closed or merged._ @lhoestq can you review when you have some time? Hi @lhoestq -- thanks so much for your review! I just added the stuff you requested to the README.md, including an example from the dataset, the table of contents, and lots of section headers with "More Information Needed" below. Let me know if there's anything else I need to do! Feel free to also merge `master` into your branch to get the latest updates for the tests ;) thanks @lhoestq - just made all the updates you requested!
Adds the Adversarial GLUE dataset: https://adversarialglue.github.io/

```python
>>> import datasets
>>> >>> datasets.load_dataset('adv_glue')
Using the latest cached version of the module from /home/jxm3/.cache/huggingface/modules/datasets_modules/datasets/adv_glue/26709a83facad2830d72d4419dd179c0be092f4ad3303ad0ebe815d0cdba5cb4 (last modified on Mon Mar  7 19:19:48 2022) since it couldn't be found locally at adv_glue., or remotely on the Hugging Face Hub.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/jxm3/random/datasets/src/datasets/load.py", line 1657, in load_dataset
    builder_instance = load_dataset_builder(
  File "/home/jxm3/random/datasets/src/datasets/load.py", line 1510, in load_dataset_builder
    builder_instance: DatasetBuilder = builder_cls(
  File "/home/jxm3/random/datasets/src/datasets/builder.py", line 1021, in __init__
    super().__init__(*args, **kwargs)
  File "/home/jxm3/random/datasets/src/datasets/builder.py", line 258, in __init__
    self.config, self.config_id = self._create_builder_config(
  File "/home/jxm3/random/datasets/src/datasets/builder.py", line 337, in _create_builder_config
    raise ValueError(
ValueError: Config name is missing.
Please pick one among the available configs: ['adv_sst2', 'adv_qqp', 'adv_mnli', 'adv_mnli_mismatched', 'adv_qnli', 'adv_rte']
Example of usage:
	`load_dataset('adv_glue', 'adv_sst2')`
>>> datasets.load_dataset('adv_glue', 'adv_sst2')['validation'][0]
Reusing dataset adv_glue (/home/jxm3/.cache/huggingface/datasets/adv_glue/adv_sst2/1.0.0/3719a903f606f2c96654d87b421bc01114c37084057cdccae65cd7bc24b10933)
100%|███████████████████████████████████████████████████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 604.11it/s]
{'sentence': "it 's an uneven treat that bores fun at the democratic exercise while also examining its significance for those who take part .", 'label': 1, 'idx': 0}
```