Update README.md

Author: VillePihlava

README.md

@@ -184,9 +184,9 @@ OSM preprocessing is done if a bash script is defined for a specific config and
 See [hsl.sh](hsl/osm-preprocessing/hsl.sh) for an example.
 
 When creating OSM preprocessing instructions you should:
-1. Name the bash file as follows: `<osm_id_of_osm_file>.sh`. Valid file names can be e.g. `hsl.sh` or `southFinland.sh`.
+1. Name the bash file as follows: `<osm_id>.sh`. Valid file names can be e.g. `hsl.sh` or `southFinland.sh`.
 2. Place the file in the `osm-preprocessing` directory of the config you want to use.
-3. Make sure that the name of the output file is the same as the input file e.g. `hsl.pbf`.
+3. Make sure that the name of the output file is the same as the input file. The file has to be named `<osm_id>.pbf`, e.g. `hsl.pbf` or `southFinland.pbf`.
 4. Make sure that you do not reuse input and output filenames in commands:
    - INCORRECT `osmfilter hsl.o5m -o=hsl.o5m ...`
    - CORRECT `osmfilter hsl.o5m -o=hsl2.o5m ...`