Added readme

README.md

+# Simplified Health Overview Plugin for LibreNMS #
+![alt text](screenshot.png "Screenshot")
+
+# Installation #
+* Copy the HealthOverview folder and contents to your LibreNMS plugin folder (/opt/librenms/html/plugins/)
+* Enable the plugin in LibreNMS (Overview -> Plugins -> Plugin Admin)
+
+# Usage #
+Access the plugin using the Plugins menu in LibreNMS (Overview -> Plugins -> HealthOverview)
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