Doc improvement for DataArray.assign_coords

[EtienneCmb]

Hi, first I wanted to say that xarray is a really fantastic library, it saves so many manipulations in my work.

I was looking for a way to add a coordinate to a DataArray and attribute it to a specific dimension. I found that the following syntax was exactly what I was looking for :

da.assign_coords(new_coordinate=('dim_to_use', values))

However, the documentation of the method was not referencing this syntax. What do you think?

[dcherian]

👍 can you send in a PR with some add text and an example?

[EtienneCmb]

Sure, I'll add it to the doc and example