Update Windows build guide to include NSIS

Author: tedwaine

docs/reference/build_guides/windows.md

@@ -18,6 +18,10 @@ Restart your machine after Visual Studio finishes installing.
 
 Follow [these instructions](downloading_qt.md)
 
+### Download and install the NSIS tool
+
+NSIS is a packaging system that lets us build xSTUDIO into a Windows installer exe file. Follow the download link on the [NSIS homepage](https://nsis.sourceforge.io/Download). This will download an installer .exe file. Run this program and follow through the steps in the installer wizard with the default installation options until you hit 'Finish'. You can close the NSIS window, it doesn't need to be running for the next steps.
+
 ### Download the VCPKG repo
 
 Start a Windows Powershell to continue these instructions, where you must run a handfull of powershell commands to build xSTUDIO. Windows Powershell is pre-installed, to start it type Powershell into the Search bar in the Start menu. You will need a location to build xSTUDIO from. We recommend making a folder in your home space, called something like 'dev', as follows: