Update release notes

This hasn't been updated much this cycle since most of the changes were
internal and/or related to the build system.

Change-Id: Id8fe6954afda7ea8e8ea78d6914fc9d5fccfe908
Reviewed-on: http://review.couchbase.org/44292
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>