OsmoSGSN: add rate counter documentation

Change-Id: I4c5d5744f961a92bc7c8888ca739d1e8a8a0be9c
diff --git a/doc/manuals/osmosgsn-usermanual.adoc b/doc/manuals/osmosgsn-usermanual.adoc
index 79c94f6..a6a7709 100644
--- a/doc/manuals/osmosgsn-usermanual.adoc
+++ b/doc/manuals/osmosgsn-usermanual.adoc
@@ -27,6 +27,8 @@
 
 include::chapters/gsup.adoc[]
 
+include::chapters/counters.adoc[]
+
 include::../common/chapters/port_numbers.adoc[]
 
 include::../common/chapters/bibliography.adoc[]