Add field for LICENSE / CITATION

We should consider adding a field for license / citation of the source datasets since not all datasets have Creative Commons License and others require mandatory citations before the content of the dataset can be used.
There can be several of these instances present in our current tasks.
For example, Concepnet handles this using a [DATA-CREDITS.md](https://github.com/commonsense/conceptnet5/blob/master/DATA-CREDITS.md) file that includes LICENSE of all source datasets.

We can add an automated script that creates a DATA-CREDITs.md as well as a bibliography file by extracting from fields from each task. The later should also help exporting all the bibliography of the paper at once ( in the future ).

Aggregating datasets from several sources without due credits seems unethical.