Ready: Erlang VM tuning doc

A quick note here — the reason we recommend enabling this flag is that long-running processes in native code that fail to contribute toward reductions on the Erlang side (NIFs and badly-behaved BIFs), a pathological case for the scheduling algorithm, result in schedulers getting stuck sleeping. This phenomenon is typically called "Scheduler Collapse" — often seen when running Bitcask in NIF IO mode, for example. The sfwi flag checks for idle schedulers and nonzero run queues on a regular interval, and on finding such a situation wakes the idle scheduler to work the run queue.