[doc] Update rule engine document

doc/source/images/src/rules_engine_class_model.txt

@@ -0,0 +1,23 @@
+@startuml
+
+package "common.py" {
+    class "Inspection" {
+    }
+    class "Issue" {
+    }
+    class "Mark" {
+    }
+    class "Error" {
+    }
+    class "Version" {
+    }
+}
+
+package "model.py" {
+    class "Model" {
+    }
+}
+
+Inspection --|> Issue
+
+@enduml

doc/source/rules_engine.rst

@@ -36,8 +36,8 @@ currently they are defined in "steps.py" file in the directory:
 ``ostack_validator/inspections/lettuce``. They are based on lettuce framework -
 bdd framework for python.
 
-Store, extend and reuse rules
------------------------------
+Store and reuse rules
+---------------------
 
 First version of Rubick project stores rules to text files and loads them to
 memory at runtime. You can add your own rules to the set using web UI, and those
@@ -51,12 +51,15 @@ by putting them in text files in directory
 ``ostack_validator/inspections/lettuce``.
 Rules file must have name in the following format::
 
-  \*.feature. 
+  \*.feature
 
 The main requirement is that all rule conditions and actions in those files must
 be written in accordance with code of rule steps in
 ``ostack-validator/inspections/lettuce/steps.py``.
 
+Extending rules
+---------------
+
 Also you can extend rules definition by adding your own steps to steps.py. As
 an example::
 
@@ -77,13 +80,12 @@ find many adapters to OpenStack services configuration data and all additional
 information collected from OpenStack nodes. After that you can use you brand
 new rule in the scenarios as described above.
 
-In ``common.py`` you can find ``Inspection``, ``Issue``, ``Mark``, ``Error`` and
-``Version`` classes for your convenience in rule defining. ``Model.py`` contains
-Openstack model based on configuration schemas.
+In module ``ostack_validator/common.py`` you can find ``Inspection``, ``Issue``,
+``Mark``, ``Error`` and ``Version`` classes for your convenience in rule
+defining. Module ``model.py`` contains Openstack model based on configuration
+schemas.
 
-putting it in directory ostack_validator/inspections/lettuce with name like 
-this: *.feature. The main requirement is that all you actions in those files 
-must be written according to the rules in steps.py.
+.. image:: image/rules_engine_class_model.png
 
 Default rule sets
 -----------------