Add configuration comment about metatile size 2 
Add configuration comment about metatile size 2 

As raised in #277, we didn't describe how to configure 512px tiles. This patch updates the comment, making it clearer how to enable 512px tiles and what additional effects that will have (i.e: `.zip` metatiles, requiring tapalcatl).
mapzen/tilequeue,tilezen/tilequeue