add known issue of reduce-during-handoff to release notes
add known issue of reduce-during-handoff to release notes

planobe/riak,authbox-lib/riak,yangchengjian/riak,planobe/riak,amyvmiwei/riak,biddyweb/riak,glickbot/riak,laurenrother/riak,xuejianhai/riak,basho/riak,DavidAlphaFox/riak,GabrielNicolasAvellaneda/riak,elkingtonmcb/riak