[docs] Add rating module introduction

1. Add the introduction of rating modules.
2. Add operating example about enable/disable and set priority
   for rating modules.

Change-Id: I3ac6786e78fcb3246dac5f8286128101a6f88656

doc/source/index.rst

@@ -10,8 +10,8 @@ Welcome to CloudKitty's developer documentation!
 Introduction
 ============
 
-CloudKitty is a Rating As A Service project aimed at translating Ceilometer
-metrics to prices.
+CloudKitty is a Rating As A Service project aimed at translating metrics
+to prices.
 
 Installation
 ============
@@ -51,14 +51,15 @@ Modules API
 
    webapi/rating/*
 
-Modules documentation
-=====================
+Rating Module Documentation
+===========================
 
 .. toctree::
    :maxdepth: 1
-   :glob:
 
-   rating/*
+   rating/introduction.rst
+   rating/hashmap.rst
+   rating/pyscripts.rst
 
 
 Indices and tables

doc/source/rating/introduction.rst

@@ -0,0 +1,50 @@
+==========================
+Rating module introduction
+==========================
+
+There are three rating modules in Cloudkitty now, including the ``noop``,
+``hashmap`` and ``pyscripts``. Only the noop rating module is just for
+testing. All modules can be enabled and disabled dynamically. Cloudkitty
+allows to run several rating modules simultaneously, and the user or
+operator can set the priority for a module. The order in which the modules
+process the data depends on their priority. The module with the highest
+priority comes first.
+
+Enable or disable module
+=======================
+
+Enable the hashmap rating module:
+
+.. code:: raw
+
+    $ cloudkitty module-enable -n hashmap
+    +---------+---------+----------+
+    | Module  | Enabled | Priority |
+    +---------+---------+----------+
+    | hashmap | True    | 1        |
+    +---------+---------+----------+
+
+Disable the pyscripts rating module:
+
+.. code:: raw
+
+    $ cloudkitty module-disable -n pyscripts
+    +-----------+---------+----------+
+    | Module    | Enabled | Priority |
+    +-----------+---------+----------+
+    | pyscripts | False   | 1        |
+    +-----------+---------+----------+
+
+Set priority
+============
+
+Set the hashmap rating module priority to 100:
+
+.. code:: raw
+
+    $ cloudkitty module-set-priority -n hashmap -p 100
+    +---------+---------+----------+
+    | Module  | Enabled | Priority |
+    +---------+---------+----------+
+    | hashmap | True    | 100      |
+    +---------+---------+----------+