docs(contributing): add CONTRIBUTING.md

closes #9 
How strongly do you feel we should adopt these git commit formatting rules? To me, this level of process/structure seems useful on a large project with a lot of contributors/committers, where a changelog might span hundreds of commits. For our tiny team, we really just need to land code quickly, and we'll all be reading each other's PRs anyway; I think this might be needless overhead. However, I could be convinced to try it and see.