Add notice to remind user to install tox
Add notice to remind user to install tox

As tox has been removed from requirements.txt, we should remind user
to install tox.

Change-Id: Id7c5dd1c01ee70a2966eec2bdef5fff18b0c3bb2

celebdor/kuryr-libnetwork,celebdor/kuryr-libnetwork,celebdor/kuryr,celebdor/kuryr-libnetwork,openstack/kuryr,openstack/kuryr,celebdor/kuryr