update docs with v0.3.1 release notes
update docs with v0.3.1 release notes

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