Add instructions for creating and installing the gem from a local checkout.
Add instructions for creating and installing the gem from a local checkout.

gravitystorm/Ruby-Mapnik,mapnik/Ruby-Mapnik,gravitystorm/Ruby-Mapnik,gravitystorm/Ruby-Mapnik,mapnik/Ruby-Mapnik,mapnik/Ruby-Mapnik,mapnik/Ruby-Mapnik,mapnik/Ruby-Mapnik,gravitystorm/Ruby-Mapnik,gravitystorm/Ruby-Mapnik,mapnik/Ruby-Mapnik,gravitystorm/Ruby-Mapnik