Add receiver subsection for Duration Arithmetic

We prefer an instance of `ActiveSupport::Duration` as a receiver
to calculate relative time like `1.minute.after(created_at)`

Author: aeroastro

README.adoc

@@ -1860,6 +1860,19 @@ Time.zone.now + 2.days
 2.days.from_now
 ----
 
+Use an instance of `ActiveSupport::Duration` as a receiver to calculate relative time.
+
+[source,ruby]
+----
+# bad - It's confusing to read
+created_at.after(1.minute)
+expiration_date.before(3.days)
+
+# good
+1.minute.after(created_at)
+3.days.before(expiration_date)
+----
+
 == Bundler
 
 === Dev/Test Gems [[dev-test-gems]]