Contribution guidelines

Please try to follow:

• this git workflow https://gist.github.com/digitaljhelms/4287848
• the numpy docstring convention https://numpydoc.readthedocs.io/en/latest/format.html
• the semver version number convention https://semver.org/

How to:

• How to commit and submit a merge request
• How to prepare the code for a release
• How to update documentation