reformat README.md.  Change specs from 'should' to 'expect'.
will return true(lucky) if left half of numbers_array is equal to right half of numbers_array.
will return false(unlucky) if sum of left half of numbers_array if NOT equal to sum of right half of numbers_array.
add note for previous commit: delete unnecessary specs.
refactor left_half & right_half variables to include inject(:+) method.
add functionality to return 'not a valid entry' if a letter is included in the input by adding if/else statement to check if input.to_i.to_s is equal to the input.