Add documentation for Effect with state dependency

[MattiJarvinen]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Feature request
[x] Documentation issue or request

What is the current behavior?

Currently Effect documentation doesn't contain example for handling state dependent effects. Since I'm not really sure where this should even be put in 4.X documentation I just leave it here.

Expected behavior:

Adding this example would have value

...
import 'rxjs/add/operator/withLatestFrom';
...
class SomeEffects {
@Effect() refreshResultsWhenInBrowseMode$ = this.actions$
    .ofType(SearchActions.CHANGE_MODE, SearchActions.CLEAR_SEARCH)
    .withLatestFrom(this.state$)
    .filter(([action, state]) => state.search.mode === SearchMode.BROWSE)
    .map(([action, state]) => state.search.query)
    .map(query => this.searchActions.search(query));

constructor(private actions$: Actions,
            private state$: Store<AppState>) { }
}

Quoted from https:/ngrx/effects/issues/49#issuecomment-248994416

[timdeschryver]

Closing this in favor of #1383.