add mention of dropping node v12 pre-built binaries
add mention of dropping node v12 pre-built binaries

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