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:

https://github.com/gocardless/business/blob/936546b2ef5ee132e1e4dd7ad9a42c521afd2e43/lib/business/calendar.rb#L106-L122

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