Updates documentation and functions to include the new dataset

Basically:

Read this before: https://github.com/CartoDB/data-services/issues/95

````
+-- Insert Mexico zip codes
+
+INSERT INTO postal_code_polygons (the_geom,adm0_a3,postal_code)
+SELECT the_geom, 'MEX', postal_cod FROM mx_codigo_postal;
````

cc @andrewxhill