Update README for arrow dependency
Update README for arrow dependency

When modifying the WRFModelGrid object to allow for idealized model
data (see 82331e89cd267b568816a358b1e0fa2a0beb9bba), a dependency on the
arrow package was added. This dependency was reflected in the setup.py
script, but not the README.

djgagne/hagelslag,djgagne/hagelslag