Improve documentation & usage examples

[slayb3ar]

Per zahlman of HN: https://news.ycombinator.com/item?id=41935591

"It would probably be helpful to show examples of what it does support, and how that complements an existing templating engine like Jinja. In the README it looks like empyt wraps an existing templating engine, but it isn't clear what that wrapping accomplishes."