Implement rate-limiting to the format `listen`

Moving rate_limit logic to the haproxy/files/_rate_limit.cfg

Change-Id: Iefc4860793a47934703eae8efdcc2f41761f5ae5
Related-Prod: PROD-24396
diff --git a/README.rst b/README.rst
index 9710e5f..61a508e 100644
--- a/README.rst
+++ b/README.rst
@@ -622,6 +622,26 @@
           - auth admin1:AdMiN123
           rate_limit_sessions: 1000
 
+Implement rate limiting, to prevent excessive requests
+using 'format: listen'
+
+.. code-block:: yaml
+
+  haproxy:
+    proxy:
+      ...
+      listen:
+        nova_metadata_api:
+          ...
+          rate_limit:
+            duration: 3s
+            enabled: true
+            requests: 60
+            track: connection
+          servers:
+            ...
+
+
 Read more
 =========