Adds arch docs to POL

Change-Id: If8ad88148e50b76c7df29d7a16ddaa58fce9f84b
Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>

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+<!--
+This file is part of OSM Policy Management module
+All Rights Reserved to Whitestack, LLC
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may
+not use this file except in compliance with the License. You may obtain
+a copy of the License at
+
+        http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+License for the specific language governing permissions and limitations
+under the License.
+
+For those usages not covered by the Apache License, Version 2.0 please
+contact: bdiaz@whitestack.com or glavado@whitestack.com
+-->
+# Policy Management module (POL)
+
+## Summary
+
+POL accomplishes the following tasks:
+
+* Configure auto scaling groups for VNFs.
+* Configure VNF alarms for VNFs.
+* Listen for MON alarms status and evaluating corresponding policies (scaling and vnf alarms).
+* Sending scaling messages to LCM when scaling policy conditions are met.
+* Calling webhooks when VNF alarm policies are met.
+
+## Overview
+
+![](assets/POL_Overview_Diagram.jpg)
+
+POL has the following components:
+
+* POL Agent: 
+  * Listens to message bus for NS LCM actions (instantiation, scaling, deletion) and configures resources accordingly.
+  * Listens to message bus for alarm notifications, evaluates policies and executes corresponding actions.
+
+## POL Agent
+
+POL subscribes to the message bus and waits for the following messages:
+
+* topic: ns - key: instantiated
+* topic: ns - key: scaled
+* topic: ns - key: terminated
+* topic: alarm_response - key: notify_alarm
+
+When a NS is created, it checks the corresponding VNFDs for scaling group descriptors or vnf alarms. It creates the corresponding
+resources in MON.
+When a NS is scaled, it does the same as above for new VDUs if scaled out. It removes orphaned resources if scaled in.
+When a NS is terminated, it deletes associated resources.
+
+When an alarm is raised, if it corresponds to a scaling policy, it stores its last status and then validates the policy,
+as policies can have N alarms related to them and can specify a boolean operator (AND/OR) to evaluate the conditions.
+If it corresponds to a VNF alarm, the associated webhook is called.
+
+Data is stored in POL database in the OSM MySQL engine.

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