Document HV version checks in NEWS

The hypervisor-version check introduced an incompatible change:
Ganeti now errors out in situations where it used to warn and
then try and hope for the best. So add a not in the section on
incompatible changes.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

NEWS

@@ -21,6 +21,9 @@ Incompatible/important changes
   candidates can ssh into other cluster nodes and all
   non-master-candidates cannot. Refer to the UPGRADE notes
   for further instructions on the creation and distribution of the keys.
+- Ganeti now checks hypervisor version compatibility before trying an instance
+  migration. It errors out if the versions are not compatible. Add the option
+  --ignore-hvversions to restore the old behavior of only warning.
 
 New features
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