Update README for a concept diagram

Author: hoisjp

AzureMonitorAction.png

@@ -2,6 +2,8 @@
 [![build-test](https://github.com/hoisjp/azure-monitor-action/actions/workflows/test.yml/badge.svg)](https://github.com/hoisjp/azure-monitor-action/actions/workflows/test.yml)
 
 This Action lets you send custom log to Azure Monitor Log Analytics workspace from a GitHub workflow.
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+![AzureMonitorActionConcept](./AzureMonitorAction.png)
 # Usage
 
 ## Prepare secrets