Add "Getting Started" docs to repo

**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