Document `next_business_day` and `previous_business_day` in the README

[roryokane]

#next_business_day and #previous_business_day are useful public methods of Business::Calendar:

business/lib/business/calendar.rb

Lines 106 to 122 in 936546b

	# Roll forward to the next business day regardless of whether the given
	# date is a business day or not.
	def next_business_day(date)
	loop do
	date += day_interval_for(date)
	break date if business_day?(date)
	end
	end
	# Roll backward to the previous business day regardless of whether the given
	# date is a business day or not.
	def previous_business_day(date)
	loop do
	date -= day_interval_for(date)
	break date if business_day?(date)
	end
	end

The README’s “Business day arithmetic” section does not document those methods. It should.

Where I think this documentation belongs in that section of the README:

• Business day arithmetic
  • ⚠️ next_business_day – documentation needed
  • ⚠️ previous_business_day – documentation needed
  • ✅ add_business_days
  • ✅ subtract_business_days
  • ✅ roll_forward
  • ✅ roll_backward
  • ✅ business_days_between

[harsh183]

These are really useful, we ended up re-implementing these since we didn't realize they existed because it's a really useful feature.