Document how multi-threading support works in GATK4

[magicDGS]

In the classic GATK, walkers had the option to be multi-thread in two different ways:

• NanoSchedulable for thread-safe map() calls.
• TreeReducible for thread-safe map() and reduce() calls.

Because now the new framework's walkers have only one apply() function, maybe the previous design is not applicable. Nevertheless, it will be useful to implement a way to allows a tool to apply the function in a multi-thread way. Is there any plan to implement something similar in GATK4?