Adding "Getting Started" documentation section.

Adds a new high-level introduction section to docs with several diagrams that help explain the intent of the library.
Oie, right you are now that you've got me reviewing it. That was a pretty sloppy writeup... also, the diagram is equally unexpressive given that it only shows uni-directional URL updates. That will be updated. In the meantime, here's my working language update. This seems better:


> In single-page web applications, we still want to provide linkable, bookmarkable, and shareable URLs to important locations in our app. Thankfully, [there are ways](http://backbonejs.org#Router) to change the URL without reloading the page.
>
> A **Router** detects changes to the page URL, and has the opportunity update application models and/or views to reflect this new URL state. Conversely, an application may call upon the Router to set the URL at key locations as a user navigates through the app.