Mention instance reservation in NEWS

Instance reservation is a new feature to Ganeti 2.14. While this
version of Ganeti will only contain basic support, it still changes
the way Ganeti thinks about instance. Hence users should be made
aware of this feature.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>