OsmoSGSN: add rate counter documentation

Change-Id: I4c5d5744f961a92bc7c8888ca739d1e8a8a0be9c
diff --git a/doc/manuals/chapters/counters.adoc b/doc/manuals/chapters/counters.adoc
new file mode 100644
index 0000000..7fbb10c
--- /dev/null
+++ b/doc/manuals/chapters/counters.adoc
@@ -0,0 +1,4 @@
+[[counters]]
+== Counters
+
+include::./counters_generated.adoc[]