added information about how to prevent a possible error during 'generate-tiles-pg' (#1432)

Added a comment within docker-compose.yml to prevent an error during `make generate-tiles-pg`

docker-compose.yml

@@ -13,6 +13,7 @@ services:
   postgres:
     image: "${POSTGIS_IMAGE:-openmaptiles/postgis}:${TOOLS_VERSION}"
     # Use "command: postgres -c jit=off" for PostgreSQL 11+ because of slow large MVT query processing
+    # Use "shm_size: 512m" if you want to prevent a possible 'No space left on device' during 'make generate-tiles-pg'
     volumes:
       - pgdata:/var/lib/postgresql/data
     networks: