Add CocoaPods deprecation in migration (#241)

Author: zorgiepoo

documentation/upgrading/index.md

@@ -10,6 +10,8 @@ We strongly recommend upgrading Sparkle to the [latest production release](//git
 
 Sparkle 2.9 (beta) no longer distributes sparkle-cli as part of the binary distribution. Please see [the sparkle-cli documentation](/documentation/sparkle-cli) if you were depending on the tool.
 
+CocoaPods distribution has been [deprecated](https://blog.cocoapods.org/CocoaPods-Specs-Repo/). Visit the [documentation](/documentation) for more information on switching to Swift Package Manager or other distribution methods.
+
 ## Upgrading to Sparkle 2.8
 
 Sparkle 2.8 introduces a refreshed UI for the update & release notes window.