Added `--cuda-mps` as useful salloc option.

Batch-Systems.md

@@ -55,6 +55,7 @@ Further useful options:
 * `--ntasks-per-node=2`: Start 2 tasks on each node
 * `--ntasks-per-core=4`: Start 4 tasks per core
 * `--cpus-per-task=2`: Each task needs 2 processors per task
+* `--cuda-mps`: Start [MPS](https://docs.nvidia.com/deploy/mps/index.html) server. Needed if multiple MPI ranks are sharing a GPU.
 
 **Note**: A new shell is launched after allocating resources.