Change README for XCTestBootstrap
Change README for XCTestBootstrap

Summary: Some of this information is out-of-date. Let's redirect to `idb` instead.

Reviewed By: zeyadsalloum

Differential Revision: D15294729

fbshipit-source-id: bfd1f6280b10578c69842585d9c477d1e472994d

facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl,facebook/FBSimulatorControl