Add a Contributor Code of Conduct

[angela-li]

Would it be possible to add a code of conduct to this Github repo? I think it's a good practice and encourages folks to feel comfortable submitting pull requests and issues. sf has one here that we can use.

As a new contributor to open source spatial, it also makes me feel welcome in the community, which is valuable.

[rsbivand]

Would https://www.gnu.org/philosophy/kind-communication.html be OK? I'm unsure about how to licence this text. I am not convinced that adults need codes, but as mileage may vary, maybe this century they do. I have always assumed that helpful contributions are welcome, but that what is helpful for whom is open to discussion, and should be discussed directly and without pretence, especially if there are clear asymmetries in oversight or experience. This in turn needs to be weighed against the inherent value in bringing in fresh contributors with their own points of view.

I'm unsure about who polices what how, so the GNU Kind text appeals more than the shorter sf code (uses please and tries to explain why, rather than using the imperative).