Add some documentation to the README around the Router

I mostly wanted to document the HTTP method overrides, but it felt weird to have that be the only text in the whole Router section, so I also wrote up a bit about basic routes and `resource`. I copied your example from the discussion in #227 for named route generation, since it covers a lot of ground and nicely illustrates the new style.

Let me know if there's anything you'd like to be covered more, or if I've gotten something wrong.

Cheers!