Add authorship policy (GenericMappingTools#726)

* Get a copy of the authorship policy from https://github.com/fatiando/contributing
* Change the project name to PyGMT
* Update AUTHORSHIP.md
* Update contribution explanations

Co-authored-by: Wei Ji <[email protected]>
Co-authored-by: Will Schlitzer <[email protected]>

AUTHORSHIP.md

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+# Authorship guidelines for academic papers and software archives
+
+First of all, we are deeply thankful to everyone who has helped make PyGMT
+what it is today. Our goal for this document is to establish guidelines
+for giving credit to contributors for their work.
+To do so, we will attempt to define:
+
+- Fair and diverse ways of providing recognition for contributors' efforts.
+- Define _contributions_ in a broad way: writing code and/or documentation,
+ providing ideas, fostering the community, etc.
+
+The following are the ways in which individuals who have contributed will be
+recognized.
+
+> **Note**: These policies are not set in stone and may be changed to
+> accommodate the growth of the project or the preferences of the community.
+
+## The `AUTHORS.md` file
+
+Anyone who has contributed a pull request to the project is welcome to add
+themselves to the `AUTHORS.md` file. This file lives in the repository and is
+packaged with distributions. This is an optional process.
+
+## Changelog for each release
+
+Every time we make a release, everyone who has made a commit to the repository
+since the previous release will be mentioned in the changelog entry. If their
+full name is available on GitHub, we will use it. Otherwise, we will use the
+GitHub handle. This is a way of saying "Thank you".
+
+## Authorship on Zenodo archives of releases
+
+Anyone who has contributed to the repository (i.e., appears on `git log`) will
+be invited to be an author on the Zenodo archive of new releases.
+
+To be included as an author, you *must* add the following to the `AUTHORS.md`
+file of the repository:
+
+1. Full name
+2. Affiliation (if omitted, we will use "Unaffiliated")
+3. ORCID (optional)
+
+The order of authors will be defined by the number of commits to the repository
+(`git shortlog -sne`). The order can also be changed on a case-by-case basis.
+
+If you have contributed and do not wish to be included in Zenodo archives,
+there are a few options:
+
+1. Don't add yourself to `AUTHORS.md`
+2. Remove yourself from `AUTHORS.md`
+3. Indicate next to your name on `AUTHORS.md` that you do not wish to be
+ included with something like `(not included in Zenodo)`.
+
+## Scientific publications (papers)
+
+We aim to write academic papers for most of our software packages. Ideally, we
+will publish updated papers for major changes or significant new components of the
+package.
+
+To be included as an author on the paper, you *must* satisfy the following
+criteria:
+
+1. Have made multiple and regular contributions to the repository, or the GMT repository, in numerous facets, such as wrapping functions, testing, and/or writing documentation.
+2. Have made non-coding contributions, including project administration and decision making.
+3. Have participated in the writing and reviewing of the paper.
+2. Add your full name, affiliation, and (optionally) ORCID to the paper. These
+ can be submitted on pull requests to the corresponding paper repository.
+3. Write and/or read and review the manuscript in a timely manner and provide
+ comments on the paper (even if it's just an "OK", but preferably more).
+
+The order of authors will be defined by the number of commits made since the
+previous major release that has an associated paper (`git shortlog
+vX.0.0...HEAD -sne`). The order of any author who hasn't made any commits will
+be decided by all authors. The order can also be changed on a case-by-case
+basis.