Add Mapbox Navigator TOS to the license

[mcwhittemore]

@danesfeder - As our work to have a landing page for Navigator has been delayed, thoughts on adding the Navigator License to this project? I think this should help clear up and close #1391.

If so, we should do this for https://github.com/mapbox/mapbox-navigation-ios as well.

cc @frederoni @akitchen @1ec5 @karussell

[danesfeder]

Thanks @mcwhittemore 👍

[karussell]

BTW: I don't think, this is legally clean:

1. with this you change the license of all the work in the repository even that of contributors that did agree to the MIT license only and
2. the MIT license under the Mapbox license does not make any sense, so you should remove this to avoid confusion