Add contributor guide

[berendt]

We need a chapter on how to make contributions with us.

GitHub workflow, commit messages, central repository for issues, ...

[berendt]

@linwalth Can you please list what you would find useful?

The Contributor Guide here is independent of the Contributor Guide for SCS.

[linwalth]

It would be nice to have a link to a prominent page where the different projects/repos of which OSISM is comprised and the way they work together are explained. - Maybe a small diagram, too.

Right now it is a bit complex and takes a lot of time to grasp what OSISM does "under the hood" and which things are connected and influence each other. To see which depends on what and where would be really helpful to get in the project.

I know that this is more of a "meta" objective but for someone who wants to contribute it is important to quickly get a grasp of the infrastructure.

Another non-meta thing which we think would be important is a list of goals and non-goals of the project to quickly make understood how generic/specific contributions have to be.