README.rst: mention possibility to give argument to local build
README.rst: mention possibility to give argument to local build

This allows to build just subset of images. Without argument,
target image list is parsed from CI settings and will include
all images used in CI, plus test suites.

Signed-off-by: Olev Kartau <daa0e368367f231b2eec38ece459024257643988@intel.com>

intel/intel-iot-refkit,jairglez/intel-iot-refkit,YinThong/intel-iot-refkit,jairglez/intel-iot-refkit,YinThong/intel-iot-refkit,intel/intel-iot-refkit,jairglez/intel-iot-refkit,ipuustin/intel-iot-refkit,mythi/intel-iot-refkit,jairglez/intel-iot-refkit,jairglez/intel-iot-refkit,ipuustin/intel-iot-refkit,YinThong/intel-iot-refkit,mythi/intel-iot-refkit,jairglez/intel-iot-refkit,intel/intel-iot-refkit,mythi/intel-iot-refkit,ipuustin/intel-iot-refkit,klihub/intel-iot-refkit,intel/intel-iot-refkit,klihub/intel-iot-refkit,klihub/intel-iot-refkit,YinThong/intel-iot-refkit,intel/intel-iot-refkit,klihub/intel-iot-refkit,YinThong/intel-iot-refkit,jairglez/intel-iot-refkit,intel/intel-iot-refkit,intel/intel-iot-refkit,klihub/intel-iot-refkit,YinThong/intel-iot-refkit,mythi/intel-iot-refkit,ipuustin/intel-iot-refkit,mythi/intel-iot-refkit,ipuustin/intel-iot-refkit,klihub/intel-iot-refkit,YinThong/intel-iot-refkit,mythi/intel-iot-refkit,mythi/intel-iot-refkit,klihub/intel-iot-refkit,ipuustin/intel-iot-refkit,ipuustin/intel-iot-refkit