Improve the README, including updating the example charm

The README (which also [displays on PyPI](https://pypi.org/project/ops/)) could be more approachable for people who want to quickly get the point of Ops and try out Ops.

Mainly, I think it would be great if the README had a tight, self-contained set of steps for making a simple charm and showing what the charm does when deployed & tested. We should be able to take advantage of Concierge and an improved Charmcraft profile (upcoming) to limit the number of steps.