Updated upgrade readme

README-upgrade.md

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+# Upgrading Apigee Edge
+
+The upgrade-edge.yml will perform an upgrade of Apigee Edge. Edge 4.16.xx is supported for upgrade
+through Edge 4.18.01. The upgrade process will be engaged when you invoke `ansible-playbook update-edge.yml`. 
+The upgrade process will be looking for two variables. 
+
+## Variables Used
+| Variable Name | Value | Description |
+| --- | --- | --- |
+| opdk_version | 4.18.01 | This indicates the target opdk version to be installed. This is also set during a clean installation of OPDK. | 
+| upgrade_from_opdk_version | 4.16.01, 4.16.05, 4.16.09, 4.17.01, 4.17.05, 4.17.09 | This is the version of opdk that you would be upgrading from | 
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