Add instructions to install GDAL to get ogr2ogr binary.

I tried to generate TopoJSON files from Natural Earth using `make`, but I was unable to generate all the files as `ogr2ogr` was not present on my system.

This adds `ogr2ogr` in the install process. I learned how to do that from the Let's Make a Map [step](http://bost.ocks.org/mike/map/#installing-tools).