Added note to whats-new about multiple missing value support
Added note to whats-new about multiple missing value support

petercable/xray,markelg/xray,jcmgray/xarray,jhamman/xray,NicWayand/xray,shoyer/xarray,pydata/xarray,shoyer/xarray,drewokane/xray,jhamman/xarray,xray/xray,shoyer/xray,jhamman/xarray,rabernat/xray,cpaulik/xray,markelg/xray,cpaulik/xray,pydata/xarray,pydata/xarray,rabernat/xray,markelg/xray,kjordahl/xray,jhamman/xarray,chunweiyuan/xarray,kjordahl/xray