Break contributing docs out into a discrete file

[dfarrell07]

I think the model of using a discrete CONTRIBUTING.markdown and README.markdown is becoming a best practice. If nothing else, I like it and am using it in a number of other projects (dfarrell07/vagrant-opendaylight, dfarrell07/puppet-opendaylight). GitHub also picks up the CONTRIBUTING.markdown file and provides a helpful link to it when raising Issues/PRs.