release notes: add KUDU-2190 and KUDU-2173 for 1.6.0
release notes: add KUDU-2190 and KUDU-2173 for 1.6.0

Change-Id: Ib5b01a77b07073f529f31e5a9fb06af9afa113be
Reviewed-on: http://gerrit.cloudera.org:8080/8703
Tested-by: Kudu Jenkins
Reviewed-by: Mike Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@apache.org>

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