Update README for video recording.
Update README for video recording.

Summary: Adds some details about how to use `fbsimctl` to record the video of a booted Simulator in Xcode 8

Reviewed By: asm89

Differential Revision: D3943878

fbshipit-source-id: ebda3af364237831b43b06a567b0011cc49a0ffd

facebook/FBSimulatorControl,bootstraponline/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,bootstraponline/FBSimulatorControl,StevenUpForever/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,plu/FBSimulatorControl,bootstraponline/FBSimulatorControl,bootstraponline/FBSimulatorControl,StevenUpForever/FBSimulatorControl,ichu501/FBSimulatorControl,plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl