Extending README in dumps with citation

[essepuntato]

It is important to add a DataCite-compliant quotable citations for each OCC dataset dump deposited with Figshare. In particular the Figshare documentation and the README file of each dataset should contains something along the lines of the following paragraphs:

This archive contains the dump of the OpenCitations Corpus (OCC, http://opencitations.net) dataset about author roles, created regularly every month.

The correct bibliographic citation to use for this dataset, compliant with DataCite guidelines (https://www.datacite.org/cite-your-data.html), is:

OpenCitations (2017). The OpenCitations Corpus dataset of all the agent roles, archived on 2017-07-25. Figshare. https://doi.org/10.6084/m9.figshare.5373235

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This issue is related to #25