hlr: Add chapter on USSD configuration

Change-Id: Ie5196ece9f340303bcb4868e3333a9698e28c3d0
diff --git a/doc/manuals/osmohlr-usermanual.adoc b/doc/manuals/osmohlr-usermanual.adoc
index 043c971..d9a4654 100644
--- a/doc/manuals/osmohlr-usermanual.adoc
+++ b/doc/manuals/osmohlr-usermanual.adoc
@@ -14,6 +14,8 @@
 
 include::chapters/subscribers.adoc[]
 
+include::chapters/ussd.adoc[]
+
 include::../common/chapters/vty.adoc[]
 
 include::../common/chapters/logging.adoc[]