Add developing docs

DEVELOPING.md

@@ -0,0 +1,30 @@
+## Developing
+
+Installing:
+
+    git clone git@github.com:mapbox/carto.git
+    npm install
+
+Test:
+
+    npm test
+
+Running the head binary:
+
+    ./bin/carto
+
+## Documentation
+
+This repository contains auto-generated documentation of the content of Carto
+that's published on MapBox.com.
+
+    git fetch origin gh-pages:gh-pages
+
+Edit `_docs/package.json` to point to the head version of [mapnik-reference](https://github.com/mapnik/mapnik-reference).
+
+    cd _docs
+    npm install
+    node generate.js
+
+Then run `jekyll serve -p 4000` to test the new site at `localhost:4000/carto`,
+git add your changes and push.