Improve new top-level README for this project

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do. We need to improve the README

Describe the solution you'd like The new README should:

• Explain the Ballista vision
• Show diagram, scheduler screenshot, and/or code example to make the README compelling
• Document the current state of Ballista
• Links to documentation and talks to learn more
• Tell people how to get involved

Describe alternatives you've considered None

Additional context None