Readme improvement to help better find controllers for stimuli generation

[DylanTao]

- Tell users which set of controllers to use (the ones under tdw_physics/tdw_physics/target_controller or physics-benchmarking-neurips2021/stimuli/generation/controllers/)

• If we want them to use tdw_physics ones, we need
  • clear instructions for where to find physion controllers (i.e maybe rename target_controller as physion_controller, or bolden target_controller to emphasize; the controller folder can easily be mistaken for the right one)
  • better pointer to command_line_args for regeneration (say it's under "physics-benchmarking-neurips2021/stimuli/generation/configs/" but not in tdw)
• If we want them to use tdw_physics ones, we need to fix
  • a clear pointer to stimuli controllers (currently the right controller is in tdw_physics/tdw_physics/target_controller and there is also a ./controllers folder)