Update contributor's guide

[jbreue16]

as per https://github.com/cadet/CADET-Project-Management/issues/91

[schmoelder]

Generally looks good. Should we start tracking the individual tasks from the (e.g. linting / auto-formatting) here or make separate issues?

See also #404

[jbreue16]

I think the linting, formatting is an issue on its own, ive added a todo to add this to the contributing.md in #404. The rest is already in place and the contributing.md only links to where more detailed descriptions can be found.

[schmoelder]

Gotcha. For reference, here is the CADET-Process Contributor's guide. I'm quite happy with how it turned out. But note, we don't have a real developer's guide so it has double duty here.

But I'm also not entirely sure yet where exactly the difference (although there certainly is one).

[jbreue16]

Thanks! Ive added the PR vs issue and branch policy sections. The rest is either already documented in the dev-guide (and i want to avoid repetition), or missing (formatting, pre-commit).

[lieres]

Consider mentioning "private" mail to [email protected] as additional contact option.