Doc Improvement: Custom reducers should clearly be encouraged/supported

[fabianeichinger]

I'd love to see a documented support for providing this navigation model a custom reducer / onNavigation method which is propagated to build in or library provided Components like NavigationCard or NavigationHeader.

The docs currently make it sound like there is only a bunch of default actions, which could be extended, but the only supported reducer is NavigationReducer. Is that the case?

[ericvicenti]

There is no restriction to the default set of actions and NavigationReducer. You can set up the application with whatever reducer you want. If you use NavigationContainer.RootContainer, then you can provide a reducer prop. When you use a component wrapped with NavigationContainer.create, then you can override the onNavigation handler for the container.

The code in this repo doesn't have this new API just yet. I'll update it soon and make it easier to experiment with.