Update CONTRIBUTING.md per new Github process.

@opscode/client-eng please check out the new description of contribution process which should make things a lot more simple. 

Once we finalize this I will: 

* Update contribution process for chef-dk, ohai, mixlib-shellout, mixlib-authentication, mixlib-cli, mixlib-config, mixlib-log and omnibus-chef to point to this document.
* Point below links to this document: 
  * https://wiki.opscode.com/display/chef/Code+Review
  * https://wiki.opscode.com/display/chef/How+to+Contribute
  * http://docs.opscode.com/community_contributions.html

Please let me know if there are any other projects that we maintain as @opscode/client-eng which should be covered under the same process.