Clarify use of gres flag for GPU images.

Resolves #18.

content/guides/running_applications/using_singularity.md

@@ -142,10 +142,17 @@ the `tensorflow-gpu` image and need the packages `nibabel` and `tables`.
 
 {{% panel theme="info" header="Run an interactive SLURM job" %}}
 {{< highlight bash >}}
-srun --pty --mem=4gb --qos=short $SHELL
+srun --pty --mem=4gb --qos=short --gres=gpu --partition=gpu $SHELL
 {{< /highlight >}}
 {{% /panel %}}
 
+{{% notice info %}}
+The `--gres=gpu --partition=gpu` options are used here as the `tensorflow-gpu` image is
+GPU enabled.  If you are using a non-GPU image, you may omit those options.  See the page
+on [submitting GPU jobs]({{< relref "/guides/submitting_jobs/submitting_cuda_or_openacc_jobs/_index.md" >}})
+for more information.
+{{% /notice %}}
+
 After the job starts, the prompt will change to indicate you're on a
 worker node.  Next, start an interactive session in the container.