Update 01-single-particle-simulations.md

Added an early callout/mention of npsim.

Author: sjdkay

_episodes/01-single-particle-simulations.md

@@ -13,6 +13,9 @@ keypoints:
 ---
 In this first episode we will go through the running of single particle events with `ddsim`, using the built-in event generator of `ddsim`. This is the quickest way so run some straightforward tests of the geometry and produce output hits in the detectors for further analysis.
 
+> You may also see reference to using `npsim` in some tutorials. We will discuss the difference between `ddsim` and `npsim`, along with cases where one should be used over the other, in the next episode of this tutorial.
+{: .callout}
+
 ## Passing options to `ddsim`
 
 The program `ddsim` is part of the DD4hep installation when it is compiled with Geant4 support. In the EIC standard environment `eic-shell` it is available and used for many simulations in the suite of continuous integration and benchmarking checks for the geometry. Simply entering `ddsim --help` will show the large wealth of options that can be passed to `ddsim` on the command line.