Add some screenshots/recordings to README to demonstrate UI

[ChandlerSwift]

Ideally these should capture the basic points of the UI and workflow:

• [ ] admin setup process/admin panel
• [ ] cloning source code
• [ ] creating attacks
• [ ] viewing attack results
• [ ] test against gold (pending #103)
• [ ] scoreboards
• [ ] ...?

(And of course this should be done with fake users, and not using real users' data!)