Add current .NET windows framework requirement to README.

README.md

@@ -17,6 +17,8 @@ need to build OpenSim before running it.
 
 # Running OpenSim on Windows
 
+You will need .NET Framework 3.5 installed to run OpenSimulator.
+
 We recommend that you run OpenSim from a command prompt on Windows in order
 to capture any errors.