Add contributing guide

This paragraph is all funky - we're not really developing "tools" (we're developing a PaaS that uses Kubernetes), and this repo doesn't contain "instrumentation", it contains the actual PaaS.  The last sentence can be rephrased as "Kubernetes is included with OpenShift for ease of development - the version we include is periodically updated.  In the future it will be possible to run OpenShift on top of an existing system"