Update 'README.md'

README.md

@@ -68,6 +68,28 @@ bin/osmosis --rb inputs/bretagne/bretagne-latest.osm.pbf  \
     --osm2graph file=outputs/rennes.mapgr writer=insa2018 map-id=0x235
 ```
 
+### Bounding box
+
+Bounding box files should contain a single line with comma-separated (**without spaces**) values:
+
+```bash
+minLat,minLon,maxLat,maxLon
+```
+
+To use it with `osmosis`:
+
+- for `mapfile-writer`, simply `cat` the file to set the `bbox` argument:
+
+```bash
+bbox=$(cat inputs/bbox/morbihan.bbox) 
+```
+
+- for `osm2graph`, it is a bit more complicated:
+
+```bash
+--bounding-box $(cat inputs/bbox/morbihan.bbox | tr "," " " | xargs printf "bottom=%f left=%f top=%f right=%f") completeWays=yes
+```
+
 ### Fixing coastline
 
 When generating mapsforge file that contains sea, you will need to fix the PBF file prior to running osmosis.