Add docs about new repo / deprecate this repo / change mapbox forward

[gmaclennan]

I see that this repo is still receiving issues and PRs. As far as I understand all development is now happening in https://github.com/react-native-mapbox/maps, but I couldn't see this documented anywhere except from buried in this closed issue.

I imagine that many users of react-native-mapbox-gl are coming here, and it is currently the repo you reach when you visit https://github.com/mapbox/react-native-mapbox-gl (the previous repo when this was officially supported by Mapbox).

@nitaliano my understanding is has moved to community ownership under https://github.com/react-native-mapbox/maps/ and you do not have the time to review issues here. Is it possible to close issues on this repo, or transfer them to the new repo? Can I help with this? I think there is still some confusion about the status of this library, and I know the explanations are in some (sometimes heated) discussions in issues, but it would be good to find a way of letting others know the status of things.

I could open a PR with an update to the README that just has:

# Deprecated

Development of this library has move to https://github.com/react-native-mapbox/maps/

[systemlevel]

@gmaclennan check out this comment at the new repository for additional context on this issue rnmapbox/maps#7 (comment)

[kristfal]

We'll do this when we have NPM publishing right for the new org. Right now, that sits with @nitaliano alone.

[kristfal]

This is now done.