Add README info with example for fetching other types of widgets

[mofojed]

With #29, we added support for fetching other types of widget by name and passing the session. An example:

from pydeephaven import Session
from deephaven_ipywidgets import DeephavenWidget
session = Session(auth_token="iris", auth_type="io.deephaven.authentication.psk.PskAuthenticationHandler")
session.run_script("""
from deephaven import empty_table
from deephaven.plot.figure import Figure
t = empty_table(100).update(["X=i", "Y=i*i"])
f = Figure().plot_xy(series_name="X vs Y", t=t, x="X", y="Y").show()
""")
display(DeephavenWidget("f", session=session))

We should add that example to the README