Add contributing guide

You don't really need Docker to develop OpenShift.  The client tools will run on any platform, the server technically runs under Mac/Windows but won't do much without Docker.  

Maybe rephrase and say "You can develop OpenShift 3 on Windows, Mac, or Linux, but you'll need Docker installed on Linux to actually launch containers".  When we have a vagrant env or a setup with boot2docker we can sort through more there.  I wouldn't call out etcd specifically - we embed it for now so it's not worth mentioning.