doc: Add Osmux documentation to OsmoMGW User Manual

Depends: osmo-gsm-manuals.git I182d94c63f7d74ef882b77be59a95b1b7d8a4e5e
Change-Id: Ie53f98777070fc00ed085646f698d20f8cf49553
diff --git a/doc/manuals/chapters/mgcp_extensions.adoc b/doc/manuals/chapters/mgcp_extensions.adoc
index a68d785..df314bc 100644
--- a/doc/manuals/chapters/mgcp_extensions.adoc
+++ b/doc/manuals/chapters/mgcp_extensions.adoc
@@ -66,3 +66,7 @@
 a=rtpmap:97 GSM-EFR/8000
 a=ptime:40
 ----
+
+=== `X-Osmux`
+
+See <<mgcp-extension-osmux>>