readme instructions for backup_restore_profile.py need to mention dependency

[chrisdavidmills]

The readme instructions for backup_restore_profile.py need to mention its dependency on utilities/adb_helper.py. It's not as easy to use as it used to be (the previous .sh script was standalone) in this regard. I wrote up instructions on using it here: https://developer.mozilla.org/en-US/Firefox_OS/Developing_Gaia/Gaia_tools_reference#Backup_and_restore_profile

[zapion]

Hi Chris, thanks for comments. Can you indicate which part we can update according to paragraph mentioned?

[chrisdavidmills]

I would just do this:

1. After the line "This script was written for backup and restore user profile." add "Note that this script has a dependency: utilities/adb_helper.py."
2. Underneath the "Usage:" subheading, update "usage: backup_restore_profile.py to "usage: python backup_restore_profile.py".
3. At the very bottom of the section, just above the "change_channel.sh" title, add "Also see usage instructions on MDN."