Add section about new chat layout

docs/usage/upgrade_guide.md

-# New Changes
+# Upgrade Guide
+
+This guide is intended to help users upgrading from older versions with the recent changes.
 
 ## Autocomplete
 
@@ -31,6 +33,12 @@ in their realname field.
 Avatars can be configured to be square or round, and additionally, you can disable the avatars in the message list as
 well.
 
+## Chat Layout
+
+The new default chat layout shows nicks in a separate row, groups messages from the same user, and shows avatars.
+
+To return to the previous layout, you can disable "Show Avatars", "Separate Nicknames" and "Right-Aligned Timestamps".
+
 ## Highlights & Unread Activity
 
 In Quassel 0.13, highlights and unread activity are managed by the core. This means you won’t ever miss a highlight.