docs: add 1.5.0 release note for thread count reduction
docs: add 1.5.0 release note for thread count reduction

Change-Id: I06939f0e01db780f86a7da16e92e5acd188b6925
Reviewed-on: http://gerrit.cloudera.org:8080/7905
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@apache.org>
Tested-by: Dan Burkert <4ef91e292a55314d2653d35ff669a6bd939ce515@apache.org>

cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu