Improve README for consistency

[geerlingguy]

After running the test on a couple different laptops, I noticed that I forgot to do a couple things to increase the test-to-test consistency between laptops:

• Disable the Screen Saver: Open System Preferences > Desktop & Screen Saver, then set the Screen Saver to 'Start after: Never'.
• Turn up brightness to 100%: This emulates the harshest conditions, e.g. when I'm working in a bright airport terminal.
• Open Terminal to Full Screen mode: This ensures the screen uses a consistent amount of power from laptop-to-laptop (if there are different desktop images, icons on desktop, etc. then the screen will use very slightly different amounts of power).

[geerlingguy]

Also: make sure the directory in which you download the script should not me in a 'synced' folder (e.g. Not in Dropbox, or in Desktop folder if you sync it with iCloud).