Ready: Document runtime_health parameters

This document is pretty basic for now, but may serve us well in the future if more runtime health parameters are added.

Ready for review @jcapricebasho @jhmullen @lukebakken 
"Riak logs warnings related to busy operating system processes and garbage collection" <= I read this three times before getting it. It seemed like a sentence fragment, until I realized you were using "logs" as a verb. How about:

"Riak will log warnings related to busy operating system processes and garbage collection"