Improve Singularity.md documentation (#349)

doc/user_guide/Singularity.md

@@ -90,3 +90,22 @@ If you wish to read further about Singularity, you can find a substantial amount
 singularity home website: https://sylabs.io/guides/3.5/user-guide/introduction.html. Including many tutorials and more 
 complex use cases.
 
+
+## Exporting visualisations on HPC's
+By default, BioDynaMo uses ParaView for visualization, which needs an X-display server to function. Since the Singularity image does not provide a graphical desktop environment and hence no X-server, there are two workarounds.
+
+### Option 1: Using xvfb
+```xvfb``` stands for X virtual frame buffer and emulates the required display server. Start a virtual frame buffer with: 
+```
+/biodynamo/util/xvfb-initd.sh start
+```
+The buffer can be managed with the arguments ```start```|```stop```|```restart```
+
+Note: Some systems may require display forwarding **before** starting the frame buffer. This can be achived through ```export DISPLAY=0```.
+
+### Option 2: Setting Paraview to work in a headless mode using the CMAKE flag
+Paraview with the osmesa or EGL backend does not require an X-server. To change the backend, you have to edit the cmake options in [1], build paraview, and replace the ParaView version that BioDynaMo installs. See also [2].
+
+[1] https://github.com/BioDynaMo/biodynamo/blob/HPC-documentation/util/build-third-party/paraview/build.sh
+
+[2] https://discourse.paraview.org/t/pvpython-offscreen-rendering-segfault/246/3