Update Readme to include screenshots or more "what does this do" language

[electblake]

For anyone new coming into the project, it can be quite confusing to get started. The project about page at https://www.dronecode.org/about is fairly broad, and being a javascript / angularjs developer I naturally want to know what I can do immediately with this project.

I'd love to see screenshots, a feature list, or some links to additional reading that can help explain what this project integrates with and what abilities it provides (or sees to provide).

Thanks :)