osmo-mgw: Add vty reference manual

add missing vty reference manual for osmo-mgw

Change-Id: If9d8cdcbbbf14b23e48af5b9ae1c56c2a38219a2
diff --git a/doc/manuals/Makefile b/doc/manuals/Makefile
new file mode 100644
index 0000000..46c97e4
--- /dev/null
+++ b/doc/manuals/Makefile
@@ -0,0 +1,6 @@
+TOPDIR = ..
+
+VTY_REFERENCE = osmomgw-vty-reference.xml
+include $(TOPDIR)/build/Makefile.vty-reference.inc
+
+include $(TOPDIR)/build/Makefile.common.inc