Starting point for documenting Clustered mode

**WIP**
:construction: 

Opening this PR as a starting point for documenting Clustered Mode. I think there are some additional thread related information here that I'm not understanding yet. 


**Goals:**
- How to use clustered mode
- implications of threads for each cluster (I assume each cluster get it's own `-t` number of threads)
- Implications of the choice of ruby implementation (do workers map to cores on JRuby or Rubinius, e.g.)
- What you should use `on_worker_boot` for

Full render of changes here: https://github.com/ctshryock/puma/blob/cluster_docs/README.md#clustered-mode

I'd like feedback and help filling some of these gaps.