[Feature request] Add a feature to dataset
This is what I have so far: 

```
import pyarrow as pa
from nlp.arrow_dataset import Dataset

aug_dataset = dataset['train'][:]
aug_dataset['new_info'] = new_info

#reformat as arrow-table
schema = dataset['train'].schema

# this line doesn't work:
schema.append(pa.field('new_info', pa.int32()))

table = pa.Table.from_pydict(
    aug_dataset,
    schema=schema
)
dataset['train'] = Dataset(table) 
```
Is there a straightforward way to add a field to the arrow_dataset, prior to performing map?
[Feature request] Add a feature to dataset 
 Is there a straightforward way to add a field to the arrow_dataset, prior to performing map? 
 This is what I have so far: 

```
import pyarrow as pa
from nlp.arrow_dataset import Dataset

aug_dataset = dataset['train'][:]
aug_dataset['new_info'] = new_info

#reformat as arrow-table
schema = dataset['train'].schema

# this line doesn't work:
schema.append(pa.field('new_info', pa.int32()))

table = pa.Table.from_pydict(
    aug_dataset,
    schema=schema
)
dataset['train'] = Dataset(table) 
```