Genericize mapshaper sample command.

README.md

@@ -17,10 +17,14 @@ Follow the steps below to use the tool to create your own cutouts. The San Franc
 ```
 curl 'https://download.geofabrik.de/north-america/us/california/norcal-latest.osm.pbf' -o ./volumes/input/latest.osm.pbf
 ```
-2. Decide what counties you want to include in your OSM cutout. Find each county in [counties-usa.geojson](counties-usa.geojson), confirm spelling, capitalization, etc... 
-3. Add all of the counties you want in your extract as a comma seperated list in the follow command, replacing `Alameda,Contra Costa,Marin,Napa,San Mateo,Santa Clara,Solano,Sonoma,San Francisco`. Set the name of the output file (replacing `bay-area-geometry.geojson` with something that makes sense). Then run it. This should create a geojson file in the `./polygons` directory. 
-```
-npx --yes mapshaper -i counties-usa.geojson -filter '"Alameda,Contra Costa,Marin,Napa,San Mateo,Santa Clara,Solano,Sonoma,San Francisco".indexOf(NAME) > -1' -dissolve2 -o ./polygons/san-francisco-bay-area.geojson geojson-type=Feature
+2. Decide which counties you want to include in your OSM cutout. Find each county in [counties-usa.geojson](counties-usa.geojson), confirm spelling, capitalization, etc...
+3. Add all the counties you want in your extract as a comma seperated list in the following command, e.g. `Alameda,Contra Costa,Marin,Napa,San Mateo,Santa Clara,Solano,Sonoma,San Francisco`. Set the name of the output file. Then run it. This should create a geojson file in the `./polygons` directory. 
+```
+npx --yes mapshaper \
+    -i counties-usa.geojson \
+    -filter '"{list-of-comma-separated-county-names}".indexOf(NAME) > -1' \
+    -dissolve2 \
+    -o ./polygons/{my-region-name}.geojson geojson-type=Feature
 ```
 4. (optional) Create a convex hull of the counties. This is useful when the group of counties don't make a solid shape. Use the geojson file created in the previous step to make a new geojson file. 
 ```