Clearer instructions for registering at Mapbox (possibly others)

[php4fan]

From the docs here:
https://github.com/leaflet-extras/leaflet-providers

Providers requiring registration
Mapbox
In order to use Mapbox maps, you must register.

That links to the generic sign-up page for Mapbox, but they offer a lot of products and it is not clear what you need to register to in order to be able to use their tiles with leaflet-providers.

I haven't looked at the other non-free providers, so I'm not sure whether the same issue may apply.

[brunob]

Feel free to provide a more precise url if needed, but i don't think it's up to us to document mapbox usage and registration :p

[brunob]

Any news about this @php4fan or should i close this one ?