add comments for running in docker

docker/Dockerfile

@@ -1,5 +1,12 @@
 # mirrord's Dockerfile
 
+# You can run like this:
+# docker run -d -v /path/to/host_dir_to_put_files:/home/mirror \
+# -v /path/to/host_dir_put_log_files:/var/log/mirrord \
+# -v /path/to/host_dir_contains_config_files:/etc/mirror ideal/mirror
+
+# Please use docker ps and docker logs to view container logs.
+
 FROM archlinux:latest
 
 MAINTAINER ideal <[email protected]>