Update docs for hero-loader

[aaronfrost]

Add docs so that people can see how to use it better.

[mattdsteele]

I'll get the ball rolling with a question...

Is hero-loader required/recommended in Angular 8? Given that every component is now a potential entry component (and modules can be loaded with import() instead of via strings), how does that change best practices for lazy-loading modules outside of a routed context?