What license is the book under?

[cjvanlissa]

Dear all,

What an excellent resource! I'm making some open educational materials on formal theory which touch upon causal inference, and I would like to base some examples on the book so that students can easily go there if they want to get more in depth on causal inference. However, I noticed that there is no license associated with the book. Would you consider adding a LICENSE file so that others know if and how they can use the book?

Best,
Caspar