Add authorship guidelines

[leouieda]

We should have clear guidelines for authorship in papers related to Fatiando projects. We could start with the Jupyter guidelines: https://github.com/jupyter/governance/blob/master/papers.md

I would change some of the terms to specifically mention that criteria apply between the last publication and now. So if an author in a previous publication hasn't made many contributions between then and now, they should not be included in the authors list. This is a way to make sure the project can be handed over to someone else when the current devs leave.

[leouieda]

Fixed in #13