[gtk+] Mention keymaps in the migration guide

[Matthias Clasen <matthiasc src gnome org>]

commit a326884df803582f101fb667d4111236f032e838
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Dec 15 07:48:36 2017 -0500

    Mention keymaps in the migration guide
    
    Just another bullet point.

 docs/reference/gtk/migrating-3to4.xml |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.xml b/docs/reference/gtk/migrating-3to4.xml
index a5b8210..db3e592 100644
--- a/docs/reference/gtk/migrating-3to4.xml
+++ b/docs/reference/gtk/migrating-3to4.xml
@@ -209,6 +209,15 @@
     </section>
 
     <section>
+      <title>Adapt to GdkKeymap API changes</title>
+      <para>
+        The way to get a keymap has changed slightly. gdk_keymap_get_default() has
+        been dropped, and gdk_keymap_get_for_display() has been renamed to
+        gdk_display_get_keymap().
+      </para>
+    </section>
+
+    <section>
       <title>Adapt to GtkHeaderBar API changes</title>
       <para>
         The gtk_header_bar_set_show_close_button() function has been renamed to