add example message to send service admins for testing

tsa/howto/ganeti.mdwn

@@ -267,7 +267,24 @@ Import procedure:
     
         ./ganeti -v -H $INSTANCE renumber-instance --ganeti-node $SPARE_NODE
 
- 5. test services by changing your `/etc/hosts`
+ 5. test services by changing your `/etc/hosts`, possibly warning
+    service admins:
+
+    > Subject: $INSTANCE IP address change planned for Ganeti migration
+    >
+    > I will soon migrate this virtual machine to the new ganeti cluster. this
+    > will involve an IP address change which might affect the service.
+    >
+    > Please let me know if there are any problems you can think of. in
+    > particular, do let me know if any internal (inside the server) or external
+    > (outside the server) services hardcodes the IP address of the virtual
+    > machine.
+    >
+    > A test instance has been setup. You can test the service by
+    > adding the following to your /etc/hosts:
+    >
+    >     116.202.120.182 $INSTANCE
+    >     2a01:4f8:fff0:4f:266:37ff:fe32:cfb2 $INSTANCE
 
  6. destroy test instance: