Mention DRBD 8.4 support in NEWS

Mention the main features of DRBD 8.4 support in the NEWS file.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

NEWS

@@ -2,6 +2,19 @@ News
 ====
 
 
+Version 2.9.0 beta1
+-------------------
+
+*(unreleased)*
+
+- DRBD 8.4 support. Depending on the installed DRBD version, Ganeti now uses
+  the correct command syntax. It is possible to use different DRBD versions
+  on different nodes as long as they are compatible to each other. This
+  enables rolling upgrades of DRBD with no downtime. As permanent operation
+  of different DRBD versions within a node group is discouraged,
+  ``gnt-cluster verify`` will emit a warning if it detects such a situation.
+
+
 Version 2.8.0 beta1
 -------------------