Put zarr in open_mfdataset engine list

Author: weiji14

xarray/backends/api.py

@@ -850,8 +850,8 @@ def open_mfdataset(
         If provided, call this function on each dataset prior to concatenation.
         You can find the file-name from which each dataset was loaded in
         ``ds.encoding['source']``.
-    engine : {'netcdf4', 'scipy', 'pydap', 'h5netcdf', 'pynio', 'cfgrib'}, \
-        optional
+    engine : {'netcdf4', 'scipy', 'pydap', 'h5netcdf', 'pynio', 'cfgrib',
+        'zarr'}, optional
         Engine to use when reading files. If not provided, the default engine
         is chosen based on available dependencies, with a preference for
         'netcdf4'.