django_on_call.urls: Add a ListView URL and associated template

Using the default template name for a ListView of OnCall instances
[1].  Also document the new view in the README.

[1]: https://docs.djangoproject.com/en/dev/ref/class-based-views/generic-display/#listview
README: Replace '...' -> '…'

There's no need to use a poor-man's ellipsis when we can use the real
thing ;).