Added release notes for 6.0 and upgrade path to CHANGELOG
Added release notes for 6.0 and upgrade path to CHANGELOG

nicholasrq/react-virtualized,cesarandreu/react-virtualized,bvaughn/react-virtualized,cesarandreu/react-virtualized,nicholasrq/react-virtualized,edulan/react-virtualized,bvaughn/react-virtualized,edulan/react-virtualized