add details about index change to changelog refs #590 mapnik/mapnik#3217 [skip ci]
add details about index change to changelog refs #590 mapnik/mapnik#3217 [skip ci]

langateam/node-mapnik,CartoDB/node-mapnik,tomhughes/node-mapnik,langateam/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,CartoDB/node-mapnik,langateam/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,CartoDB/node-mapnik,mapnik/node-mapnik