message: Add robot_config envvar to docs
diff: @@ -94,7 +94,7 @@ $ make install
  $ python -c 'import opentrons; print(opentrons.__version__)' # > prints version
  # run with a real robot
  -$ make dev
  +$ OT_ROBOT_CONFIG=[your_robot_serial_number] make dev
  # run with a virtual robot
  $ ENABLE_VIRTUAL_SMOOTHIE=true make dev
  