Add known issue with leveldb read block error stat to release notes
Add known issue with leveldb read block error stat to release notes

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