Add contribution guidelines

[jellepoland]

• how to format commit message
• when is it allowed to push to the main branch
• some words about code formatting

Example: https://github.com/JuliaControl/ControlSystems.jl/blob/master/CONTRIBUTING.md

[1-Bart-1]

Commit messages should be in imperative form, and say what the commit "does".

[ufechner7]

For a draft, see #121