Feature: Add CONTRIBUTING.md scaffold

@dshaw suggested we should add a CONTRIBUTING.md file that outlines directions in contributing.
https://github.com/nodeconf/Adventure-UNCONF/issues/21

One example provided by github themselves was puppet
https://raw.githubusercontent.com/puppetlabs/puppet/master/CONTRIBUTING.md
or from factory_girl_rails
https://raw.githubusercontent.com/thoughtbot/factory_girl_rails/master/CONTRIBUTING.md

Should the scaffolding be a general outline? Something likes this?
1.) Check past issues to see if the feature request has been made.
2.) Something about suggesting/requesting in the project's issues page
3.) Fork the project
4.) Review the code
5.) branch code from master, add feature, test feature, push branch aka make pull request

:+1: or :-1: