One thing to include: an explanation of how much background someone should have before contributing. You might think this is intimidating, but it is really helpful because it lets a potential contributor disambiguate whether they should

A) Just start contributing, sure in the knowledge that they aren't missing some piece of background that others would consider obvious.

B) Go off and search for a good book/course to learn about X, where X is a googlable phrase that your contributing.md just told them.