Introducing a CONTRIBUTING guide

No - not yet, at least. While there are [tools the provide dependency management](https://github.com/nitrous-io/goop) and [a number of strategies that those who depend on us can follow](http://nathany.com/go-packages/), out of the box, Go provides no way to depend on any version but HEAD. In fact, "go get" is an internal tool, and there's no way to give it any sort of metadata at all.