OsmoPCU: add rate counter documentation

Change-Id: Ieb4e1dab415a70ded5c65c21752dca497856e96f
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[]