added docker instructions

Author: audiracmichelle

README.md

@@ -120,7 +120,7 @@ docker pull nsaph/climate_types_raster2polygon
 # to generate county aggregations
 docker run -v <test_path>:/app/data/output/climate_types_raster2polygon nsaph/climate_types_raster2polygon
 # to generate zcta aggregations
-docker run -v <test_path>:/app/data/output/climate_types_raster2polygon nsaph/climate_types_raster2polygon -C shapefile_polygon_name=zcta
+docker run -v <test_path>:/app/data/output/climate_types_raster2polygon nsaph/climate_types_raster2polygon --cores=1 -C shapefile_polygon_name=zcta
 ```
 
 ### If you want to build your own image use