add User Story section to CONTRIBUTING.md

add User Story section with recommendation to stimulate discussion around [this](https://www.pivotaltracker.com/story/show/78270810) user story:
"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."
update CONTRIBUTING.md

added guidance on capturing non-user-story items, e.g,. ideas, comments, questions, concerns to CONTRIBUTING.md
Merge pull request #67 from NYULibraries/chore/add-user-story-contribution-questions

add User Story section to CONTRIBUTING.md