Add link to WOF license file

public/attribution.md

@@ -4,4 +4,4 @@
    * [OpenStreetMap](http://www.openstreetmap.org/copyright) © OpenStreetMap contributors under [ODbL](http://opendatacommons.org/licenses/odbl/)
    * [OpenAddresses](http://openaddresses.io) under a [Creative Commons Zero](https://github.com/openaddresses/openaddresses/blob/master/sources/LICENSE) public domain designation
    * [GeoNames](http://www.geonames.org/) under [CC-BY-3.0](https://creativecommons.org/licenses/by/2.0/)
-   * [WhosOnFirst](http://whosonfirst.mapzen.com)
+   * [WhosOnFirst](http://whosonfirst.mapzen.com) under [various licenses](https://github.com/whosonfirst/whosonfirst-data/blob/master/LICENSE.md)