Dataset configuration
Following discussions from https://github.com/huggingface/datasets/pull/5331

We could have something like `config.json` to define the configuration of a dataset.

```json
{
    "data_dir": "data"
    "data_files":  {
        "train": "train-[0-9][0-9][0-9][0-9]-of-[0-9][0-9][0-9][0-9][0-9]*.*"
    }
}
```

we could also support a list for several configs with a 'config_name' field.

The alternative was to use YAML in the README.md.

I think it could also support a `dataset_type` field to specify which dataset builder class to use, and the other parameters would be the builder's parameters. Some parameters exist for all builders like `data_files` and `data_dir`, but some parameters are builder specific like `sep` for csv.

This format would be used in `push_to_hub` to be able to push multiple configs.

cc @huggingface/datasets 

EDIT: actually we're going for the YAML approach in README.md
Originally we also though about adding it to the YAML part of the README.md:

```yaml
builder_config:
  data_dir: data
  data_files:
  - split: train
    pattern: "train-[0-9][0-9][0-9][0-9]-of-[0-9][0-9][0-9][0-9][0-9]*.*"
```

Having it in the README.md could make it easier to modify it in the UI on HF, and for validation on commit From internal discussions we agreed to go with the YAML approach, since it's the one that seems more appropriate to be modified by a human on the Hub or locally (while JSON e.g. for models are usually created programmatically). Current format:
```yaml
builder_config:
  data_files:
  - split: train
    pattern: data/train-*
```