added a third project which impacts Windows-only dev

docs/build.md

@@ -7,8 +7,11 @@ Perspex requires Visual Studio 2015 to build on Windows.
 ### Install GTK Sharp
 
 To compile the full project under windows, you must have [gtk-sharp](http://www.mono-project.com/download/#download-win) installed. However, if you're 
-not interested in building the cross-platform bits you can simply unload the Perspex.Cairo and 
-Perspex.Gtk project in Visual Studio.
+not interested in building the cross-platform bits you can simply unload these projects from Visual Studio:
+
+ - Perspex.Cairo
+ - Perspex.Cairo.RenderTests
+ - Perspex.Gtk
 
 ### Clone the Perspex repository