No provider for NotificationsService with documented usage

[asnelling]

Importing SimpleNotificationsModule according to the documentation (below):

@NgModule({
    imports: [BrowserModule, SimpleNotificationsModule],
    declarations: [AppComponent],
    bootstrap: [AppComponent]
})
export class AppModule { }

results in a "No provider for NotificationsService" error after 313ca4d

The following resolves the error, so it seems the documentation should be updated.

@NgModule({
    imports: [BrowserModule, SimpleNotificationsModule.forRoot()],  // call `.forRoot()` method
    declarations: [AppComponent],
    bootstrap: [AppComponent]
})
export class AppModule { }

I'm gonna put in a PR shortly with updated docs.