Add instructions for `xarray.open_mfdataset()` to documentation

[mx-moth]

Prompted by #53, the documentation never states that the ems accessor on datasets is available on any dataset, including those opened with xarray.open_mfdataset(). The documentation should make it clear that emsarray.open_dataset() is not required in order to access emsarray functionality. All that is required is that the emsarray module has been imported at some point (in order to register the xarray accessor).

Example code:

import emsarray
import xarray

dataset = xarray.open_mfdataset(...)
dataset.ems.plot()