add srun to SLURM user guide
Adding documentation on the srun command (taken from the DUBii slides: https://du-bii.github.io/module-1-Environnement-Unix/seance2/slides/#65
Adding documentation on the srun command (taken from the DUBii slides: https://du-bii.github.io/module-1-Environnement-Unix/seance2/slides/#65