Docs for using custom elements with Marko

- [ ] Unrecognized tag and [`html-elements.json`](https://github.com/mlrawlings/custom-elements-everywhere/blob/master/libraries/marko/src/html-elements.json)
- [ ] Attributes passed as properties

See [`custom-elements-everywhere/marko`](https://github.com/mlrawlings/custom-elements-everywhere/tree/master/libraries/marko/src) for example implementation and tests until docs are complete.