Document VaadinRouterView

[haijian-vaadin]

Document VaadinRouterView once it's implemented, see #422

Note: this ticket is about the document in vaadin-router.
A new ticket will be made to update the general Vaadin Router documentation in flow-and-components-documentation.

[vlukashov]

This is about updating the Vaadin Route docs at https://github.com/vaadin/vaadin-router/tree/master/docs, but not yet about publishing them to https://vaadin.github.io/vaadin-router/vaadin-router/demo/#vaadin-router-lifecycle-callbacks-demos, as that would require a new npm release first.