add a paragraph on how to use router outside of a component defining router

[Alexander-Taran]

issue in router repo for reference:
aurelia/router#412

For a beginer it was not obvious how to get router inside a vm
other than configureRouter
and using that one w/o routes ended up badly

Got to add it in meaningful way.

Most common use case is to navigate in code
I'd go with that..

if you want to use router.navigate... inside a component that does not define routes...
...
some text giving an example of
import {}

@autoinject / js example of DI

and using a method..

reference to DI section of docs

mention other possible use cases of an injected router class..