Add initial OsmoMGW manual

Change-Id: I1b4ff96309d280c6a63105a6e06a82ec2e7b6908
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[]