Islands have been part of the OpenMapTiles schema for more than five years (see commits d664a2fe and 01db9a4e and issue #68), but they're not documented in [the layer definition](https://github.com/openmaptiles/openmaptiles/blob/master/layers/place/place.yaml) or [on openmaptiles.org](https://openmaptiles.org/schema/#place). This PR fixes that by updating `layers/place/place.yaml`.