documentation missing how to split a dataset
this seems to work for my specific problem:

`self.train_ds, self.test_ds, self.val_ds = map(_prepare_ds, ('train', 'test[:25%]+test[50%:75%]', 'test[75%:]'))` 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 added a small guide [here](https://github.com/huggingface/nlp/tree/master/docs/splits.md) that explains how to split a dataset. It is very similar to the tensorflow datasets guide, as we kept the same logic. Thanks a lot, the new explanation is very helpful!

About using train_test_split from sklearn: I stumbled across the [same error message as this user ](https://github.com/huggingface/nlp/issues/147 )and thought it can't be used at the moment in this context. Will check it out again.

One of the problems is how to shuffle very large datasets, which don't fit into the memory. Well, one strategy could be shuffling data in sections. But in a case where the data is sorted by the labels you have to swap larger sections first. 
 We added a way to shuffle datasets (shuffle the indices and then reorder to make a new dataset).
You can do `shuffled_dset = dataset.shuffle(seed=my_seed)`. It shuffles the whole dataset.
There is also `dataset.train_test_split()` which if very handy (with the same signature as sklearn).

Closing this issue as we added the docs for splits and tools to split datasets. Thanks again for your feedback !
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)
