Clarify licence

[simonpoole]

Given that this style is one of the popular, if not the most popular, starting points for building a map style with OpenStreetMap data, it would seem to be a good idea to clarify which, if any, licence applies to the visual design of maps created with this style (analog to https://github.com/mapbox/mapbox-gl-styles/blob/master/LICENSE.md).

Further, some thought should be given to grandfathering previous, potentially unlicensed use.