chore: change README to minimal contributor-focused version (#851)

- Replaces the full usage documentation in README with a minimal version following the same pattern as the [Go SDK README](https://github.com/Unleash/unleash-go-sdk)
- Removes some of the documentation that now (when the [PR](Unleash/unleash-documentation#82) is merged) lives in the [docs site](https://docs.getunleash.io/sdks/node) 
- Makes README focus on: quick start, requirements, and contributing (local dev, tests, benchmarking, linting, building)
- Removes stale Code Climate badge

Author: dharmadeveloper108