TOX: Document install_command usage
TOX: Document install_command usage

This patch documents in tox.ini the change introduced by change-id
Ic9a6ac412a334710eb5e45935cd301ca80a5edb9 where we use "install_command"
to prevent installing dependencies without constraints.

We document it to prevent people from thinking that just adding the
constraints in "deps" is enough and removing the "install_command" line.

Change-Id: Iccd84cb5d4db78e4953d58a05392c61b08988d4e

mahak/cinder,openstack/cinder,mahak/cinder,openstack/cinder