documentation missing how to split a dataset
Currently you can indeed split a dataset using `ds_test = nlp.load_dataset('imdb, split='test[:5000]')` (works also with percentages).

However right now we don't have a way to shuffle a dataset but we are thinking about it in the discussion in #166. Feel free to share your thoughts about it.

One trick that you can do until we have a better solution is to shuffle and split the indices of your dataset:
```python
import nlp
from sklearn.model_selection import train_test_split

imdb = nlp.load_dataset('imbd', split='test')
test_indices, val_indices = train_test_split(range(len(imdb)))
```

and then to iterate each split:
```python
for i in test_indices:
    example = imdb[i]
   ...
```

I am trying to understand how to split a dataset ( as arrow_dataset). 
I know I can do something like this to access a split which is already in the original dataset : 

`ds_test = nlp.load_dataset('imdb, split='test') `

But how can I split ds_test into a test and a validation set (without reading the data into memory and keeping the arrow_dataset as container)?
I guess it has something to do with the module split :-) but there is no real documentation in the code but only a reference to a longer description: 

> See the  [guide on splits](https://github.com/huggingface/nlp/tree/master/docs/splits.md)  for more information.

But the guide seems to be missing.

To clarify: I know that this has been modelled after the dataset of tensorflow and that some of the documentation there can be used [like this one](https://www.tensorflow.org/datasets/splits). But to come back to the example above: I cannot simply split the testset doing this: 
`ds_test = nlp.load_dataset('imdb, split='test'[:5000]) `
`ds_val = nlp.load_dataset('imdb, split='test'[5000:])`

because the imdb test data is sorted by class (probably not a good idea anyway)

documentation missing how to split a dataset 
 I am trying to understand how to split a dataset ( as arrow_dataset). 
I know I can do something like this to access a split which is already in the original dataset : 

`ds_test = nlp.load_dataset('imdb, split='test') `

But how can I split ds_test into a test and a validation set (without reading the data into memory and keeping the arrow_dataset as container)?
I guess it has something to do with the module split :-) but there is no real documentation in the code but only a reference to a longer description: 

> See the  [guide on splits](https://github.com/huggingface/nlp/tree/master/docs/splits.md)  for more information.

But the guide seems to be missing.

To clarify: I know that this has been modelled after the dataset of tensorflow and that some of the documentation there can be used [like this one](https://www.tensorflow.org/datasets/splits). But to come back to the example above: I cannot simply split the testset doing this: 
`ds_test = nlp.load_dataset('imdb, split='test'[:5000]) `
`ds_val = nlp.load_dataset('imdb, split='test'[5000:])`

because the imdb test data is sorted by class (probably not a good idea anyway)
 
 Currently you can indeed split a dataset using `ds_test = nlp.load_dataset('imdb, split='test[:5000]')` (works also with percentages).

However right now we don't have a way to shuffle a dataset but we are thinking about it in the discussion in #166. Feel free to share your thoughts about it.

One trick that you can do until we have a better solution is to shuffle and split the indices of your dataset:
```python
import nlp
from sklearn.model_selection import train_test_split

imdb = nlp.load_dataset('imbd', split='test')
test_indices, val_indices = train_test_split(range(len(imdb)))
```

and then to iterate each split:
```python
for i in test_indices:
    example = imdb[i]
   ...
```
