added a code example for setting custom responsive breakpoints

Maybe a needless inclusion but comes out of spending 45 minutes wondering why my custom breakpoints weren't triggering when they should be as a result of copy-pasting the code out below the ``RESPONSIVE_BREAKPOINTS`` header. Maybe it would make sense to just use one dictionary name instead of two?
Merge pull request #11 from jbergantine/patch-1

added a code example for setting custom responsive breakpoints