Add user stories to DESIGN.md

[amyjko]

The problem

@jlumbroso suggested that the DESIGN.md should have user stories that describe the various operations that different stakeholders would do to interact with the website to achieve various goals.

Your idea

User stories to add:

• A conference program chair creating a venue
• A journal editor creating a venue
• A scholar volunteering to review for a venue
• A scholar bidding on a paper