Comparisons with xarray-regrid

[aulemahal]

The xarray-regrid package seems to have better performance in some areas where I don't see why xESMF should be different.

Two issues :

• Improve performance of conservative routine xarray-contrib/xarray-regrid#42
• Comparison with xESMF xarray-contrib/xarray-regrid#48

Probably that our chunking management is not the best.

Also, I think it would be good to mention xarray-regrid in our doc and to point out the differences. xarray-regrid only handles rectilinear grids, but it is much more lightweight and handles out-of-memory grids, so we could direct users there if it fits their needs.