MB-8013 Mention detailed rebalance progress feature in CHANGES.

Change-Id: I23a81f634773e55935c59612fb3df4d9ed3cdce5
Reviewed-on: http://review.couchbase.org/25911
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>