Since we can't put comments in the package.json file
this file fills that gap.

Dependencies needing comments:

 "mapnik-reference"

 We use our own fork because we need advertisement of "marker-multi-policy"
 which is only in the reference of mapnik-2.1.1. Can be reverted to upstream
 once this pull request is released:
 https://github.com/mapnik/mapnik-reference/pull/41