fix(docs): Add line setting proper permissions in README

There has been some confusion after we switched the Docker images to use
a non-root user. It's now fairly easy for permissions to be set
incorrectly.

This line in the example script should make things right.

README.md

@@ -252,6 +252,9 @@ set -x
 # create directories
 mkdir /code /data
 
+# set proper permissions. make sure the user matches your `DOCKER_USER` setting in `.env`
+chown 1000:1000 /code /data
+
 # clone repo
 cd /code
 git clone https://github.com/pelias/docker.git