Add Windows headless installer help.

README-windows.txt

@@ -4,6 +4,24 @@ Mosquitto for Windows
 Mosquitto for Windows comes in 64-bit and 32-bit flavours. All dependencies are
 provided in the installer.
 
+Installing
+----------
+
+Running the installer will present the normal type of graphical installer. If
+you want to install without starting the graphical part of the installer, you
+can do so by running it from a cmd prompt with the `/S` switch:
+
+```
+mosquitto-2.0.12-install-windows-x64.exe /S
+```
+
+You can override the installation directory with the `/D` switch:
+
+```
+mosquitto-2.0.12-install-windows-x64.exe /S /D=:\mosquitto
+```
+
+
 Capabilities
 ------------