add User Story section to CONTRIBUTING.md

@ckassel @dlovins @NYULibraries/hydra 

Please add your thoughts, comments, recommendations to 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."
How about something like: 

"Ideas, comments, questions, or concerns that cannot be easily described in a user story can be raised in IRC, meetings with product owners, emails to the Ichabod group alias, or captured as or as half-baked story-stubs in Pivotal Tracker."