Document shardable client

Changes.md

@@ -31,6 +31,13 @@ end
 ```
 - **Process Heartbeat** - each Sidekiq process will ping Redis every 5
   seconds to give a summary of the Sidekiq population at work.
+- **Targetable Client** - Sidekiq::Client instances can target different
+  Redis instances, allowing very large Sidekiq installations to scale by
+  targeting different jobs to different Redis instances.
+```ruby
+client = Sidekiq::Client.new(ConnectionPool.new { Redis.new })
+client.push(...)
+```
 - New Chinese, Greek, Swedish and Czech translations for the Web UI.
 - Updated most languages translations for the new UI features.
 - **Remove official Capistrano integration** - this integration has been

lib/sidekiq/client.rb

@@ -25,6 +25,13 @@ module Sidekiq
       @chain
     end
 
+    # Sidekiq::Client normally uses the default Redis pool but you may
+    # pass a custom ConnectionPool if you want to shard your
+    # Sidekiq jobs across several Redis instances (for scalability
+    # reasons, e.g.)
+    #
+    #   Sidekiq::Client.new(ConnectionPool.new { Redis.new })
+    #
     def initialize(redis_pool = Sidekiq.redis_pool)
       @redis_pool = redis_pool
     end