Missing Code of Conduct

[BobinMathew]

This repo is missing a Code of Conduct.
Cant I give a PR ?

[duckontheweb]

Yes, please! That's definitely something that we should have.

I think it would be good to make it consistent with the Code of Conduct in the STAC spec repo, but I'm also open to discussion of other ideas.

[duckontheweb]

Closed via #399