initial 4.0 upgrade notes

4.0-Upgrade.md

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+# Upgrading to Sidekiq 4.0
+
+Sidekiq 4.0 contains a completely redesigned core with much higher
+performance.  See my blog for [an overview of Sidekiq 4's higher performance](http://www.mikeperham.com/2015/10/14/optimizing-sidekiq/).
+
+To upgrade cleanly:
+
+* Upgrade to the latest Sidekiq 3.x.
+  `gem 'sidekiq', '< 4'`
+  Fix any deprecation warnings you see.  Then upgrade to 4.0.x.
+
+## What's New
+
+* Sidekiq no longer uses Celluloid.  If your application code uses Celluloid,
+  you will need to pull it in yourself.
+
+* `redis-namespace` has been removed from Sidekiq's gem dependencies. If
+  you want to use namespacing ([and I strongly urge you not to](http://www.mikeperham.com/2015/09/24/storing-data-with-redis/)), you'll need to add the gem to your Gemfile:
+```ruby
+gem 'redis-namespace'
+```
+
+* **Redis 2.8.0 or greater is required.**  Redis 2.8 was released two years
+  ago and contains **many** useful features which Sidekiq couldn't
+  leverage until now.  Redis 3.0.3 or greater is recommended for large
+  scale use.
+
+* Sidekiq requires more Redis connections per process due to a change in
+  how jobs are fetched from Redis.  You must have a minimum of
+  `concurrency + 2` connections in your pool or Sidekiq will exit.
+  When in doubt, let Sidekiq size the connection pool for you.