usermanual: add chapter about mgcp endpoints

The manual does not say much about the endpoint naming, even though
osmo-mgw is oriented at RFC 3435 the endpoint names are specific to
osmo-mgw. In particular the E1/T1 endpoints and their overlapping need
some explaination

Change-Id: I16265eb667221959a69f33701d024bd8d9b22040
diff --git a/doc/manuals/osmomgw-usermanual.adoc b/doc/manuals/osmomgw-usermanual.adoc
index 36d4049..c4660f5 100644
--- a/doc/manuals/osmomgw-usermanual.adoc
+++ b/doc/manuals/osmomgw-usermanual.adoc
@@ -18,6 +18,8 @@
 
 include::{srcdir}/chapters/configuration.adoc[]
 
+include::{srcdir}/chapters/mgcp_endpoints.adoc[]
+
 include::{srcdir}/chapters/mgcp_extensions.adoc[]
 
 include::./common/chapters/osmux/osmux.adoc[]