warn about FLAT_NODES w/ UPDATES incompatibility (#282)

See Zverik/regional#17

README.md

@@ -232,6 +232,8 @@ docker run \
     import
 ```
 
+Warning: enabling `FLAT_NOTES` together with `UPDATES` only works for entire planet imports (without a `.poly` file).  Otherwise this will break the automatic update script. This is because trimming the differential updates to the specific regions currently isn't supported when using flat nodes.
+
 ### Benchmarks
 
 You can find an example of the import performance to expect with this image on the [OpenStreetMap wiki](https://wiki.openstreetmap.org/wiki/Osm2pgsql/benchmarks#debian_9_.2F_openstreetmap-tile-server).