Improve new top-level README for this project

**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

Start with the repository's top-level README and gather the project vision, current state, documentation and talk links, and contribution guidance. Add the requested diagram, scheduler screenshot, and/or code example; done means the README covers each listed topic and makes the project compelling.

Written by the indexing model from the issue text.