osmo-mgw: Add vty reference manual

add missing vty reference manual for osmo-mgw

Change-Id: If9d8cdcbbbf14b23e48af5b9ae1c56c2a38219a2
diff --git a/doc/manuals/vty/mgw_vty_additions.xml b/doc/manuals/vty/mgw_vty_additions.xml
new file mode 100644
index 0000000..a4c675e
--- /dev/null
+++ b/doc/manuals/vty/mgw_vty_additions.xml
@@ -0,0 +1,2 @@
+<vtydoc xmlns='urn:osmocom:xml:libosmocore:vty:doc:1.0'>
+</vtydoc>