Added example of GPU ID list to command line arg table.

README.rst

@@ -173,7 +173,7 @@ Optional command line arguments
 Argument name          Type      Description
 ====================== ========= ===========
 ``-n``                 integer   number of times to run the input file. This option can be used to run a series of models, e.g. to create a B-scan with 60 traces: ``(gprMax)$ python -m gprMax user_models/cylinder_Bscan_2D.in -n 60``
-``-gpu``               flag/list flag to use NVIDIA GPU or list of NVIDIA GPU device ID(s) for specific GPU card(s).
+``-gpu``               flag/list flag to use NVIDIA GPU or list of NVIDIA GPU device ID(s) for specific GPU card(s), e.g. ``-gpu 0 1``
 ``-restart``           integer   model number to start/restart simulation from. It would typically be used to restart a series of models from a specific model number, with the ``-n`` argument, e.g. to restart from A-scan 45 when creating a B-scan with 60 traces: ``(gprMax)$ python -m gprMax user_models/cylinder_Bscan_2D.in -n 15 -restart 45``
 ``-task``              integer   task identifier (model number) when running simulation as a job array on `Open Grid Scheduler/Grid Engine <http://gridscheduler.sourceforge.net/index.html>`_. For further details see the `parallel performance section of the User Guide <http://docs.gprmax.com/en/latest/openmp_mpi.html>`_
 ``-mpi``               integer   number of Message Passing Interface (MPI) tasks, i.e. master + workers, for MPI task farm. This option is most usefully combined with ``-n`` to allow individual models to be farmed out using a MPI task farm, e.g. to create a B-scan with 60 traces and use MPI to farm out each trace: ``(gprMax)$ python -m gprMax user_models/cylinder_Bscan_2D.in -n 60 -mpi 61``. For further details see the `parallel performance section of the User Guide <http://docs.gprmax.com/en/latest/openmp_mpi.html>`_