Update license to bookcorpus dataset card (#3526)

* Update README.md

Adding license, as best I can figure out from the relevant links.

* Add smashwords ToS as licensing information

Co-authored-by: Albert Villanova del Moral <8515462+albertvillanova@users.noreply.github.com>

Commit from https://github.com/huggingface/datasets/commit/db2e5b5ea26a6103833b6f6c7671b29ee4594969

README.md

@@ -153,7 +153,9 @@ 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 https://www.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
 
@@ -165,10 +167,8 @@ The data fields are the same among all splits.
     month = {December},
     year = {2015}
 }
-
 ```
 
-
 ### Contributions
 
-Thanks to [@lewtun](https://github.com/lewtun), [@richarddwang](https://github.com/richarddwang), [@lhoestq](https://github.com/lhoestq), [@thomwolf](https://github.com/thomwolf) for adding this dataset.
+Thanks to [@lewtun](https://github.com/lewtun), [@richarddwang](https://github.com/richarddwang), [@lhoestq](https://github.com/lhoestq), [@thomwolf](https://github.com/thomwolf) for adding this dataset.