Merge pull request #606 from anildigital/master

Add Countdown Latch example.

Author: Petr Chalupa

lib/concurrent/atomic/count_down_latch.rb

@@ -72,6 +72,29 @@ module Concurrent
   #   with its work. A `CountDownLatch` can be used only once. Its value cannot be reset.
   #
   # @!macro count_down_latch_public_api
+  # @example Waiter and Decrementer
+  #   latch = Concurrent::CountDownLatch.new(3)
+  #
+  #   waiter = Thread.new do
+  #     latch.wait()
+  #     puts ("Waiter released")
+  #   end
+  #
+  #   decrementer = Thread.new do
+  #     sleep(1)
+  #     latch.count_down
+  #     puts latch.count
+  #
+  #     sleep(1)
+  #     latch.count_down
+  #     puts latch.count
+  #
+  #     sleep(1)
+  #     latch.count_down
+  #     puts latch.count
+  #   end
+  #
+  #   [waiter, decrementer].each(&:join)
   class CountDownLatch < CountDownLatchImplementation
   end
 end