Add initial OsmoMGW manual

Change-Id: I1b4ff96309d280c6a63105a6e06a82ec2e7b6908
diff --git a/doc/manuals/Makefile b/doc/manuals/Makefile
index 46c97e4..b092e66 100644
--- a/doc/manuals/Makefile
+++ b/doc/manuals/Makefile
@@ -1,5 +1,9 @@
 TOPDIR = ..
 
+ASCIIDOC = osmomgw-usermanual.adoc
+ASCIIDOC_DEPS = chapters/*.adoc
+include $(TOPDIR)/build/Makefile.asciidoc.inc
+
 VTY_REFERENCE = osmomgw-vty-reference.xml
 include $(TOPDIR)/build/Makefile.vty-reference.inc