Add authorship policy

[seisman]

Description of proposed changes

The aurthorship policy is copied from https://github.com/fatiando/contributing/blob/master/AUTHORSHIP.md, with tiny changes.

Fixes #691

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[willschlitzer]

"We aim to write academic papers for most of your software packages" Shouldn't that be "most of our software packages" instead?

[willschlitzer]

@GenericMappingTools/python In preparation for the v0.3 release, is the plan to have @leouieda as first author and @PaulWessel as last author, and then all of the contributors listed on AUTHORS.md in the order of numbers of commits? That makes the most sense to me, but I also don't feel strongly about a particular order.

[michaelgrund]

@GenericMappingTools/python In preparation for the v0.3 release, is the plan to have @leouieda as first author and @PaulWessel as last author, and then all of the contributors listed on AUTHORS.md in the order of numbers of commits? That makes the most sense to me, but I also don't feel strongly about a particular order.

I think that's the best way to handle the order @willschlitzer since in most scientific papers the order represents some kind of "workload" 😅.

[michaelgrund]

I'm fine with the changes.

[weiji14]

and then all of the contributors listed on AUTHORS.md in the order of numbers of commits? That makes the most sense to me, but I also don't feel strongly about a particular order.

Number of commits isn't perfect, but as long as we communicate about this in good faith, I think we'll be ok. Specifically would like to mention Liam's work on the ROSES and upcoming Scipy2021 stuff which is super valuable but not tracked on git. So this metric should serve as a rough guideline and not be set in stone.