Add boundaries `import-data` info to Readme (#1215)

* Add boundaries info to Readme

* make NE usage less specific - it may change later.

Co-authored-by: Yuri Astrakhan <yuriastrakhan@gmail.com>
Co-authored-by: Tomas Pohanka <TomPohys@gmail.com>

README.md

@@ -101,7 +101,7 @@ Now start up the database container.
 make start-db
 ```
 
-Import external data from [OpenStreetMapData](http://osmdata.openstreetmap.de/), [Natural Earth](http://www.naturalearthdata.com/) and [OpenStreetMap Lake Labels](https://github.com/lukasmartinelli/osm-lakelines).
+Import external data from [OpenStreetMapData](http://osmdata.openstreetmap.de/), [Natural Earth](http://www.naturalearthdata.com/) and [OpenStreetMap Lake Labels](https://github.com/lukasmartinelli/osm-lakelines). Natural Earth country boundaries are used in the few lowest zoom levels.
 
 ```bash
 make import-data