Add contributing guide

I may be wrong, but I'm pretty sure you need to clone the repo into github.com/openshift/origin, and then you can manipulate your git remotes appropriately. Otherwise I think all the go import statements will be off.

I typically do this:

go get github.com/openshift/origin
cd $GOPATH/src/github.com/openshift/origin
git remote rename origin upstream
git remote add origin git@github.com:<YOUR_GITHUB_USERNAME/origin