Update README.md with with hybrid MPI+OpenMP parallelism

```

> mpirun -np [n] [executable] data/paramfile.txt

```

When CPU hybrid MPI+OpenMP parallelism is requested, you have to select the number of threads by setting **--cpus-per-task=** in your bash script, then add the following lines:

```

> export OMP_NUM_THREADS=${SLURM_CPUS_PER_TASK}

> export OMP_PLACES=cores

> export OMP_PROC_BIND=close

```

Then, to run the code in order to fully fill all the available cores in the node please add **--ntasks-per-socket=** in your bash script and then run:

```

> mpirun -np [n] --bind-to core --map-by ppr:${SLURM_NTASKS_PER_SOCKET}:socket:pe=${SLURM_CPUS_PER_TASK} -x OMP_NUM_THREADS [executable] data/paramfile.txt