Add link to utfgrid-spec.
|@@ -54,7 +54,7 @@ One row in `metadata` is **suggested** and, if provided, may enhance performance|
|**left, bottom, right, top**. Example of the full earth: `-180.0,-85,180,85`.|
|Several additional keys are supported for tilesets that implement|
|-UTFGrid-based interaction. See `interaction.md`.|