UPGRADING.md in slack-ruby-client-0.12.0 vs UPGRADING.md in slack-ruby-client-0.13.0

@@ -1,8 +1,24 @@
 Upgrading Slack-Ruby-Client
 ===========================
 
+### Upgrading to >= 0.13.0
+
+#### Recommended Async Library
+
+The RealTime client now supports [async-websocket](https://github.com/socketry/async-websocket), which is actively maintained and is now the recommended library.
+
+See [#219](https://github.com/slack-ruby/slack-ruby-client/pull/219) for implementation details.
+
+#### Async Library Threading and Reactor Changes
+
+The RealTime celluloid-io implementation now uses a `Thread` per client. Previous versions used an `Actor`.
+
+The faye-websocket implementation with EventMachine will attempt to shutdown EventMachine via `EventMachine.stop` upon shutdown if a reactor wasn't already running.
+
+See [#224](https://github.com/slack-ruby/slack-ruby-client/pull/224) for more information.
+
 ### Upgrading to >= 0.9.0
 
 #### Changes in How the RTM Client Connects
 
 The RealTime client now automatically chooses either [rtm.start](https://api.slack.com/methods/rtm.start) or [rtm.connect](https://api.slack.com/methods/rtm.connect) to open a connection. The `rtm.connect` method is a newer, preferred method, which serves connection purposes and returns some basic team info. The `rtm.start` method additionally returns a lot of data about the team, its channels, and members, and is required to use the full `Slack::RealTime::Stores::Store` storage class.
@@ -35,10 +51,10 @@
 Slack::RealTime::Client.config do |config|
   config.start_options[:no_unreads] = true # keeps config.start_options[:request] intact
 end
 ```
 
-See [#136](https://github.com/slack-ruby/slack-ruby-client/pull/136) for more details.
+See [#136](https://github.com/slack-ruby/slack-ruby-client/pull/136) for more information.
 
 ### Upgrading to >= 0.6.0
 
 #### Changes to API Response Data