add more help to the permissions troubleshooting

README.md

@@ -283,4 +283,10 @@ Troubleshooting
   SQLALCHEMY_DATABASE_URI with `sqlite:////...` with the full path to the data/webpush.db file.
   (Why does the relative path of `sqlite:///...` not work for a relative path?)
 
-* Another potential problem with "unable to open database file" is the permissions on the directory or file with the DB, as set on the local volume that matches the docker `/app/instance/data` directory. Note that the user id in the Dockerfile is set to 1000; the `id` command will tell you the uid for your user, which should match.
+* Another potential problem with "unable to open database file" is the
+  permissions on the directory or file with the DB, as set on the local volume
+  that matches the docker `/app/instance/data` directory. Note that the user id
+  in the Dockerfile is set to 1000; the `id` command will tell you the uid for
+  your user, which should match. Then, user with uid 1000 should have write
+  permissions to both the file and the directory, and have execute permissions
+  to the whole path to that directory.