SUPPORT.md: Add missing support lifetime information

Dates are from Wei:

  Supported-Until:

  $ date --date '2017-12-13 + 18 months' +%F
  2019-06-13

  Security-Support-Until:

  $ date --date '2019-06-13 + 18 months' +%F
  2020-12-13

CC: Lars Kurth <lars.kurth@citrix.com>
CC: Juergen Gross <jgross@suse.com>
CC: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

SUPPORT.md

@@ -11,8 +11,8 @@ for the definitions of the support status levels etc.
 
     Xen-Version: 4.10
     Initial-Release: 2017-12-13
-    Supported-Until: TBD
-    Security-Support-Until: TBD
+    Supported-Until: 2019-06-13
+    Security-Support-Until: 2020-12-13
 
 # Feature Support