[libhandy] doc: Document the API changes

[Alexander Mikhaylenko <alexm src gnome org>]

commit e1a58a0360ef6acbd5a7e6251bcb01e35208b225
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Thu Jul 9 13:33:45 2020 +0200

    doc: Document the API changes

 doc/hdy-migrating-0-0-to-1.xml | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/doc/hdy-migrating-0-0-to-1.xml b/doc/hdy-migrating-0-0-to-1.xml
index 955a87f0..a84c4aa9 100644
--- a/doc/hdy-migrating-0-0-to-1.xml
+++ b/doc/hdy-migrating-0-0-to-1.xml
@@ -129,6 +129,22 @@
       </para>
     </refsect3>
 
+    <refsect3>
+      <title>Adapt to HdyHeaderGroup API changes</title>
+      <para>
+        The #HdyHeaderGroup object has been largely redesigned, most of its
+        methods changed, see its documentation to know more.
+      </para>
+      <para>
+        The child type is now #HdyHeaderGroupChild, which can represent either
+        a #GtkHeaderBar, a #HdyHeaderBar, or a #HdyHeaderGroup.
+      </para>
+      <para>
+        The “focus” property has been replaced by #HdyHeaderGroup:decorate-all,
+        which works quite differently.
+      </para>
+    </refsect3>
+
     <refsect3>
       <title>Adapt to HdyLeaflet API changes</title>
       <para>