Add "Getting Started" docs to repo

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I want to be able to spin up a scheduler and executor and learn about the CLI and Python bindings with as little effort as possible so I can start playing with Ballista

Describe the solution you'd like
Add a getting started guide.

Describe alternatives you've considered
None

Additional context
None