Adding license information for Openbookcorpus (#3525)

* Update README.md

* Update datasets/bookcorpusopen/README.md

Co-authored-by: Quentin Lhoest <42851186+lhoestq@users.noreply.github.com>

Commit from https://github.com/huggingface/datasets/commit/cb6e8e74b93e468bd8c6539ff4544e5e75eb30f0

README.md

@@ -160,7 +160,9 @@ The data fields are the same among all splits.
 
 ### Licensing Information
 
-[More Information Needed](https://github.com/huggingface/datasets/blob/master/CONTRIBUTING.md#how-to-contribute-to-the-dataset-cards)
+The books have been crawled from smashwords.com, see their [terms of service](https://www.smashwords.com/about/tos) for more information.
+
+A data sheet for this dataset has also been created and published in [Addressing "Documentation Debt" in Machine Learning Research: A Retrospective Datasheet for BookCorpus](https://arxiv.org/abs/2105.05241)
 
 ### Citation Information
 
@@ -178,4 +180,4 @@ The data fields are the same among all splits.
 
 ### Contributions
 
-Thanks to [@vblagoje](https://github.com/vblagoje) for adding this dataset.
+Thanks to [@vblagoje](https://github.com/vblagoje) for adding this dataset.