Added explanation of Composite provider
Added explanation of Composite provider

mojodna/TileStache,Kosta-Github/TileStache,Kosta-Github/TileStache,MetGIS/TileStache,robpvn/Vector-Tile-Research,mapzen/TileStache,Kosta-Github/TileStache,SEL-Columbia/TileStache,robpvn/Vector-Tile-Research,Billups/TileStache,TileStache/TileStache,moskvax/TileStache,mapzen/TileStache,Blechhirn/TileStache,robpvn/Vector-Tile-Research,Blechhirn/TileStache,jbants/TileStache,SEL-Columbia/TileStache,TileStache/TileStache,mapzen/TileStache,moskvax/TileStache,Blechhirn/TileStache,Billups/TileStache,TileStache/TileStache,mpuig/TileStache,SEL-Columbia/TileStache,jbants/TileStache,Billups/TileStache,jbants/TileStache,MetGIS/TileStache,mpuig/TileStache,mojodna/TileStache,moskvax/TileStache,MetGIS/TileStache