Elaborate contributing guidelines

[ashwinvis]

https://vergauwenthomas.github.io/MetObs_toolkit/MetObs_documentation_full.html

I think this is very short. According to JOSS and I agree

There should be clear guidelines for third-parties wishing to:

Contribute to the software

Report issues or problems with the software

Seek support

See here for some general guidelines

https://www.pyopensci.org/python-package-guide/documentation/repository-files/contributing-file.html

cf: openjournals/joss-reviews#5916

[Zeitsperre]

Hi @vergauwenthomas, I'm going to also jump in and agree that the contribution guidelines aren't clear. These should be found in the top-level of the project as well as be somewhere in the documentation (maybe near the developer documentation).

This is an example from one of the projects I develop/maintain: https://github.com/CSHS-CWRA/RavenPy/blob/master/CONTRIBUTING.rst (No need to follow this to the letter, but more for inspiration)

[vergauwenthomas]

A CONTRIBUTING file is added to the repo + is rendered in the documentation.