Enhance README

[volbap]

It would be cool to improve this diagram:

By adding the view controllers in the scene, and exposing how they can, for example, get all the users, create a post, update a comment, etc.
In other words, I'd like to expose the interactions between view controllers and repositories, which is the goal of the library.

Also, we need to give instructions on how to initialize a backend with your own configuration, for instance:

extension NSURLSessionBackend {
    static func myAppBackend() -> NSURLSessionBackend {
        let headers = [HTTPHeader(field: "Accept", value: "application/json"), HTTPHeader(field: "Content-Type", value: "application/json")]
        let configuration = NSURLSessionBackendConfiguration(basePath: "https://myapp.com/", httpHeaders: headers)
        return NSURLSessionBackend(configuration: configuration)
    }
} 

[volbap]

I started a refactor of the README file and its diagrams, so that it's less like "telling a story" but rather straight to the point and more user-friendly.