The permissions on the user_secrets file are too open, adjust this so that
after using pw-token-gen.py it sets the file to be 0600 for
user_secrets.yml and the backup tar file that is created. Additionally,
add a note in the README to recommend adjusting the permissions when not
utilising the pw-token-gen.py
Change-Id: I90ffacd83a89a92f48cf160e5b351e1254e9c73a
Closes-Bug: #1461997