Establish user guidelines

[sskutnik]

Develop documentation for collaborators looking to identify and use curriculum resources. Specific documentation should include:

• Overview of the basic repository concept (i.e., what is there, how it is granulated, etc.)
• How to locate and acquire content, including identifying dependency content, associated media (images, assignments, evaluations, etc.)
• How to contribute new content or revise existing nodes

[katyhuff]

These should be in a file called contributors.md : https://help.github.com/articles/setting-guidelines-for-repository-contributors/ .