Add migration guide from 1.1.8 to 1.2.0

UPDATING.md

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+# Upgrade / Migration Guide
+
+## Version 1.1.8 to 1.2.0
+
+### Notable Changes
+This release is all about channel options. Here is the full [changelog](https://github.com/ably/ably-ruby/blob/main/CHANGELOG.md)
+
+* Channel options were extracted into a seperate model [ChannelOptions](https://github.com/ably/ably-ruby/blob/main/lib/ably/models/channel_options.rb). However it's still backward campatible with `Hash` and you don't need to do make any adjustments to your code
+
+* The `ChannelOptions` class now supports `:params`, `:modes` and `:cipher` as options. Previously only `:cipher` was available
+
+### Breaking Changes
+
+* Changing channel options with `Channels#get` is now deprecated in favor of explicit options change
+
+  1. If channel state is attached or attaching an exception will be raised
+  2. Otherwise the library will emit a warning
+
+For example, the following code
+```
+  client.channels.get(channel_name, new_channel_options)
+```
+
+Should be changed to:
+```
+  channel = client.channels.get(channel_name)
+  channel.options = new_channel_options
+```