Adding license information for Openbookcorpus
The MIT license seems to be for the crawling code, no ? Then maybe we can also redirect users to the [terms of smashwords.com](https://www.smashwords.com/about/tos) regarding copyrights, in particular the paragraph 10 for end-users. In particular it seems that end users can download and use the content "for their personal enjoyment in any reasonable non-commercial manner in compliance with copyright law" and the smashwords end-users agreement.

It should be the same for https://github.com/huggingface/datasets/pull/3526 as well May I merge this one ? _The documentation is not available anymore as the PR was closed or merged._
Not entirely sure, following the links here, but it seems the relevant license is at https://github.com/soskek/bookcorpus/blob/master/LICENSE