Update README.md

Example of usage:

```python
from datasets import load_dataset

dataset = load_dataset("Andron00e/CIFAR10-custom")
splitted_dataset = dataset["train"].train_test_split(test_size=0.2)
```

README.md

@@ -19,3 +19,4 @@ configs:
   - split: train
     path: data/train-*
 ---
+