Add ELN contributor guidelines

[JanCBrammer]

Suggestions so far:

• commit title is meaningful => git history search
• commit description is helpful => helps the reviewer to understand the changes
• code is up-to-date with the latest developments of the target branch (rebased to it or whatever) => ⏩ :-merge for linear history is favored
• added code is linted
• tests are passing (at least locally)
• if the changes are visible to the end-user, video or screenshots should be added to the PR => helps with user testing
• testing coverage improvement is more than welcome
• CHANGELOG : add a bullet point on top (optional: reference to github issue/PR )
• clarify which dependencies are added, if any