Add note for additional packages.

content/applications/user_software/using_singularity.md

@@ -132,6 +132,11 @@ Alternatively, instead of building an image from scratch, you can start with an
 image as the base for your Dockerfile (i.e. `FROM unlhcc/spades`)
 and add any additional packages you desire.
 
+{{% notice warning %}}
+The following method only works for installing packages via `pip` that have no system-level dependencies.
+If you require packages that are installed via `yum` or `apt`, you will need to create a  [custom image]({{< relref "#create-a-custom-image" >}}).
+{{% /notice %}}
+
 Unfortunately it's not possible to create one image that has every
 available Python package installed for logistical reasons.  Images are
 created with a small set of the most commonly-used scientific packages,