Add a contributing.md

[johanhelsing]

@NavidK0 Just wondering how you prefer things to be done?

Things like:

• Is breaking changes ok (bumping major version of course)? Or do you prefer to avoid this as much as possible?
• Now that I get write access, do you still prefer changes to go through PRs?
• Other things?

Might make sense to put this in a contributing.md.