Update README.md to include ndjson-cli suggestion

Most software in the ecosystem is unable to write arbitrary data to the tippecanoe object, so it might be helpful to show how users can make these GeoJSON extensions work. #685

README.md

@@ -617,6 +617,12 @@ the filename or name specified using `--layer`, like this:
 }
 ```
 
+If your source GeoJSON only has `minzoom`, `maxzoom` and/or `layer` within `properties` you can use [ndjson-cli](https://github.com/mbostock/ndjson-cli/blob/master/README.md) to move them into the required `tippecanoe` object by piping the GeoJSON like this:
+
+```sh
+ndjson-map 'd.tippecanoe = { minzoom: d.properties.minzoom, maxzoom: d.properties.maxzoom, layer: d.properties.layer }, delete d.properties.minzoom, delete d.properties.maxzoom, delete d.properties.layer, d'
+```
+
 Geometric simplifications
 -------------------------