Add contributing guide

I meant 'origin', because that is what `go get` will set up by default. I think Clayton's point is that it would be best to keep this as simple as possible, and we accomplish that here by doing the minimum to add our own fork to the remotes on the repo.

So per this line of thinking, we'd have:

    $ go get github.com/openshift/origin
    $ cd $GOPATH/src/github.com/openshift/origin
    $ git remote add <username> git@github.com:<username>/origin