Merge Contributing Guidelines with IOData Guidelines

[PaulWAyers]

We need to harmonize the contributing guidelines here with those from IOData.
https://github.com/tovrstra/iodata/blob/1032c13485a2dba0f8298617028145d806d5dc1d/CONTRIBUTING.rst

We should also, eventually, carefully document the development protocol (how to set up releases, GH actions, etc.) and harmonize it across repositories.

[tovrstra]

The current contributing guide (in this repo) gives a good overview, and requires little technical background. That seems good, so I believe we should keep that format. I'd suggest adding more technical step-by-step instructions in different Markdown files, which can be referenced from the current contributing guide. I'll take a first stab at it after #4.