Add contributing guide

That could work, although did you mean "openshift" instead of "origin"?

FWIW, my workflow is typically like this, using hub:

git clone -o upstream openshift/origin
git remote add -p origin <my gh login>

But that could easily be adapted to

git clone -o openshift openshift/origin
git remote add -p <my gh login> <my gh login>

And then I could "git fetch openshift" and "git push -u <my gh login> <my feature branch"