add optimization tips, closes #1103 (#1104)
add optimization tips, closes #1103 (#1104)


jhamman/xray,markelg/xray,pydata/xarray,rabernat/xray,xray/xray,jhamman/xarray,chunweiyuan/xarray,jcmgray/xarray,markelg/xray,shoyer/xarray,rabernat/xray,jhamman/xarray,markelg/xray,pydata/xarray,shoyer/xarray,pydata/xarray,jhamman/xarray,shoyer/xray