add conda install instructions

Author: stephaniegott

ipyleaflet/leaflet.py

@@ -334,8 +334,9 @@ class Velocity(Layer):
     except ModuleNotFoundError:
         raise ImportError(
             'The `xarray` module (required for use of the Velocity layer) is'
-            'not installed. Run "$ pip install xarray" to install the latest'
-            'version.'
+            'not installed. Run "$ conda install -c conda-forge xarray dask'
+            'netCDF4 bottleneck" or "$ pip install xarray" to install the '
+            'latest version.'
         )
 
     _view_name = Unicode('LeafletVelocityView').tag(sync=True)