Create CONRIBUTING.md with correct content

[sphinxc0re]

The current CONTRIBUTING.md is more like a CODE-OF-CONDUCT.md. Which is good because now we don't have to figure out one by ourselves 😁

The CONTRIBUTING.md should only contain details for developers and non-developers on how to contribute to the project.

For devs this would mean a technical guide on how to set up the environment and a narrow explanation on which directories of the project contain which part of the code.

For non-devs this would mean a guide on how to formulate issues, how to report bugs and how to request a feature.