Update "Build your own navigator" docs for Navigation View API refactor

[brentvatne]

[keith-kurak]

I was about to open an issue about documenting the new required addListener param for addNavigationHelpers() but then saw this. Would this issue include the "Custom Navigators" (https://reactnavigation.org/docs/custom-navigators.html#addnavigationhelpers) docs and thus addNavigationHelpers()? With that additional information, we MobX folks should have enough to go on in order to integrate the new react-navigation with our stores. Thanks!

[brentvatne]

@ericvicenti probably has more to say about that

[ericvicenti]

this is what I've done so far: https://next.reactnavigation.org/docs/custom-navigators.html

Happy to hear feedback on this, or create more docs that you think are appropriate.