Add mask writing example.
Add mask writing example.

kapadia/rasterio,perrygeo/rasterio,perrygeo/rasterio,youngpm/rasterio,kapadia/rasterio,clembou/rasterio,johanvdw/rasterio,brendan-ward/rasterio,snorfalorpagus/rasterio,sgillies/rasterio,johanvdw/rasterio,njwilson23/rasterio,youngpm/rasterio,youngpm/rasterio,clembou/rasterio,njwilson23/rasterio,perrygeo/rasterio,njwilson23/rasterio,brendan-ward/rasterio,johanvdw/rasterio,brendan-ward/rasterio,clembou/rasterio,kapadia/rasterio