mention number of contributors to v0.9.0
mention number of contributors to v0.9.0

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