Add note about service mode incompatibility with > XP


git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4833 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/README.txt b/README.txt
index db5cb90..a18cbf0 100644
--- a/README.txt
+++ b/README.txt
@@ -66,7 +66,8 @@
 
 The Windows version of TigerVNC should work on Windows 2000 and later.  On
 Windows XP and later, winvnc may not work if the Fast User Switching or Remote
-Desktop features are in use.
+Desktop features are in use.  Running winvnc as a service does not (yet) work
+with Windows Vista or later.
 
 
 Unix/Linux-Specific (not Mac)