Add contributions guidelines

[IAmMarcelJung]

Contribution guidelines would be helpful for people who are new to FABulous and also to generalize the process for issues and pull requests. This could of course get out of hand featurewise (with bots etc.) but for a start some simple rules and guidelines would suffice (e.g. when to start an issue vs when to start a discusson or when to create a PR to merge into main vs merge into the 2.0 branch, etc).