Upgrade migration docs

Author: bezin

UPGRADE-6.0.md

@@ -3,4 +3,5 @@
 Several major changes in version 6.0:
  - Requires PHP 8.1
  - The factory has been removed. Use `new` to construct your FeedIO instance: `new \FeedIo\FeedIo($client, $logger)`
- - Feed IO comes no longer bundled with a default HTTP Client, but uses HTTPlug instead. To continue using Guzzle, please require `php-http/guzzle7-adapter`.
+ - Feed IO comes no longer bundled with a default HTTP client, but uses HTTPlug instead. To continue using Guzzle, please require `php-http/guzzle7-adapter`.
+ - Feed IO does no longer set a custom user agent. However, HTTP clients ususally add a default themselves. If the feed you want to read requires a specific user agent, please configure your HTTP client accordingly, before you inject it into Feed IO.