[libhandy] doc: Document derivability changes

[Alexander Mikhaylenko <alexm src gnome org>]

commit 407bf0cdb370aa528ff6b187c7eab99129b9a992
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Tue Jul 14 22:02:56 2020 +0200

    doc: Document derivability changes

 doc/hdy-migrating-0-0-to-1.xml | 12 ++++++++++++
 1 file changed, 12 insertions(+)
---
diff --git a/doc/hdy-migrating-0-0-to-1.xml b/doc/hdy-migrating-0-0-to-1.xml
index a84c4aa9..bc96ffd7 100644
--- a/doc/hdy-migrating-0-0-to-1.xml
+++ b/doc/hdy-migrating-0-0-to-1.xml
@@ -98,6 +98,18 @@
       </para>
     </refsect3>
 
+    <refsect3>
+      <title>Adapt to derivability changes</title>
+      <para>
+        Some widgets are now final, if your code is deriving from them, use
+        composition instead.
+      </para>
+      <para>
+        Affected widgets:
+        #HdySqueezer, #HdyViewSwitcher, #HdyViewSwitcherBar
+      </para>
+    </refsect3>
+
     <refsect3>
       <title>HdyFold has been removed</title>
       <para>