Add release notes entry.

doc/arm/notes.xml

@@ -269,6 +269,13 @@
 	  BIND 8 when processing an IXFR stream.  'no-change' deltas
 	  will now trigger a fallback to AXFR as the recovery mechanism.
 	</para>
+	<para>
+	  BIND 9 will no longer build on platforms that doesn't have
+	  proper IPv6 support.  BIND 9 now also requires non-broken
+	  POSIX-compatible pthread support.  Such platforms are
+	  usually long after their end-of-life date and they are
+	  neither developed nor supported by their respective vendors.
+	</para>
       </listitem>
     </itemizedlist>
   </section>