Require our fork of mapnik-reference to get marker-multi-policy

See https://github.com/Vizzuality/grainstore/issues/36

package.json

@@ -36,7 +36,7 @@
   },
   "dependencies": {
     "underscore": "~1.3.3",
-    "mapnik-reference": "~5.0.0",
+    "mapnik-reference": "git://github.com/CartoDB/mapnik-reference.git#cdb-5.0",
     "xml2js": "~0.1.13"
   },
   "devDependencies": {

package.json.README

@@ -0,0 +1,11 @@
+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