Add README for XCTestBootstrap
Add README for XCTestBootstrap

Reviewed By: nqmtuan

Differential Revision: D3133228

fb-gh-sync-id: 3963ac4252f50dd710b71711b57d555cefec5378
fbshipit-source-id: 3963ac4252f50dd710b71711b57d555cefec5378

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