Add upgrade notes to release readme

RELEASE.md

@@ -12,10 +12,22 @@ This release also includes:
 * libvips 8.16.0
 * ffmpeg 7.1
 
-Changelog for 4.3.2snap3: Let it snow! ❄️
+## Changelog for 4.3.2snap3: Let it snow! ❄️
 
 * Adds snow to the Mastodon UI between Christmas and New Year! (Thanks to [Roni Laukkarinen](https://github.com/ronilaukkarinen)!)
 * Admin account is created during setup
 * Remove the `status.char-counter` option and rename `status.char-limit` to `status.length`
 * Additional improvements were added to the `mastodon-server.setup` script
 * Mastodon Bird UI has been updated to 2.1.1
+
+## Upgrade notes
+
+Due to the changes described above, you may need to manually change the `status.length` option after the upgrade, e.g:
+
+    snap set mastodon-server status.length=5000
+
+However, the new default value of `status.length=1000` doubles Mastodon's default value of 500 and should be sufficient for most users.
+
+You can also clean up the old snap variables with:
+
+    snap unset mastodon-server status.char-limit status.char-counter