If you're interested in contributions from the community, then it's good to discuss what kinds of contributions you want, and what would constitute a "valuable" contribution. You don't want folks going off and doing work to make a contribution that you won't value, not do you want someone's well-considered contribution to be rejected because they didn't follow some unwritten guidelines.

It would be good to start by reading Setting guidelines for repository contributors. The article includes examples of good CONTRIBUTING files as well as some additional reading.