add User Story section to CONTRIBUTING.md

@ckassel @dlovins @NYULibraries/hydra 

Please add your thoughts and recommendations as comments on this PR. 

This PR is for [this](https://www.pivotaltracker.com/story/show/78270810) chore. Description below:
"As an Ichabod team member, I want to be clear on the scope of my role in recommending, influencing and/or otherwise contributing to user stories so I can help define the direction of Ichabod."