Add module import and providers to auth-guard docs

[ribomation]

Append to the code snippet about auth-guard how to import and configure providers of the new AngularFireAuthGuard. Something like

@NgModule({
  imports: [
    RouterModule.forChild(routes),
    AngularFireAuthModule,
  ],
  providers: [
    AngularFireAuthGuard
  ],
  exports: [RouterModule]
})
export class MyRoutingModule {}

[KingDarBoja]

Unless I am missing something here, there isn't much to be added besides importing the AngularFireAuthModule in some module where the AngularFireAuthGuard is required: https://github.com/angular/angularfire/blob/master/docs/auth/router-guards.md

[jamesdaniels]

Closing as outdated since we're revamping the docs for the new version.