[docs] Add tilejson URL

[nvkelso]

Way back in #73 we added support for TileJSON, but that isn't documented under: https://mapzen.com/documentation/vector-tiles/use-service/.

http://tile.mapzen.com/mapzen/vector/v1/tilejson/mapbox.json?api_key=mapzen_api_key

[nvkelso]

See: f4f1892.

[nvkelso]

Related: https://github.com/mapzen/tile-tasks/issues/297

[nvkelso]

Completed in f4f1892.