NEWS update - offline instance state

Signed-off-by: Agata Murawska <agatamurawska@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

NEWS

 News
 ====
 
+Version 2.6.0 beta 1
+--------------------
+
+*(unreleased)*
+
+- Deprecated ``admin_up`` field. Instead, ``admin_state`` is introduced,
+  with 3 possible values -- ``up``,``down`` and ``offline``.
+
 Version 2.5.0 rc2
 -----------------