Write up an Overiew on the Readme.md

[dsummersl]

I'd love to see three things:

• A summary of what this project (a dataviz tool to explore NC accident data)
• Long term goals (links to milestones). I think this is currently: milestone two (filtering/data exploration), milestone three (link up with the Raleigh group), milestone four (make it NC wide!), milestone 5 (uhm...add more data!)
• Use cases: "city planners and bike advocates want to know where to focus their efforts" (ugh, so vague!).

[dsummersl]

Here's the original source data with the user stories brainstormed back in January: https://docs.google.com/document/d/1TJBwka1wiwf2OLMlS5jjxF1aWhkX7VTuREEQFzNvjxw/edit#heading=h.7a9a38kcgqy0