stubdom: document that pty usage should be disabled

diff -r 8e2ae477718e stubdom/README
@@ -18,6 +18,8 @@
 - comment the disk statement:
 
 #disk = [  'file:/tmp/install.iso,hdc:cdrom,r', 'phy:/dev/sda6,hda,w', 'file:/tmp/test,hdb,r' ]
+
+- disable anything related to dom0, like pty serial assignments
 
 
 Create /etc/xen/stubdom-hvmconfig (where "hvmconfig" is the name of your HVM