Add release notes for 1.8.0

Change-Id: I15b0ce686c5e69648fe09a18ca82b9bf54cab837
Reviewed-on: http://gerrit.cloudera.org:8080/11647
Reviewed-by: Dan Burkert <danburkert@apache.org>
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <aserbin@cloudera.com>
1 file changed