Add docs for calculating previous/next business time

This is a common question answered by business-hours gems, so we should
show how to answer it.

README.md

@@ -96,6 +96,12 @@ Biz.time(2, :hours).after(Time.utc(2015, 12, 25, 9, 30))
 # Calculations can be performed in seconds, minutes, hours, or days
 Biz.time(1, :day).after(Time.utc(2015, 1, 8, 10))
 
+# Find the previous business time
+Biz.time(0, :hours).before(Time.utc(2016, 1, 8, 6))
+
+# Find the next business time
+Biz.time(0, :hours).after(Time.utc(2016, 1, 8, 20))
+
 # Find the amount of business time between two times
 Biz.within(Time.utc(2015, 3, 7), Time.utc(2015, 3, 14)).in_seconds