Create test_canary.md

Author: victorhuangwq

diagnostics/test_canary.md

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+# Test Feature in Canary
+
+The latest runtime changes are first shipped in the WebView2 runtime in the Edge Canary Browser. If you would like to test the new changes with your application, here's a suggested way to do so:
+
+1. Download [Edge Canary](https://www.microsoft.com/en-us/edge/download/insider)
+2. Run the following command in a PowerShell with Administrative Priviledges:  
+  `REG ADD HKCU\Software\Policies\Microsoft\Edge\WebView2\ChannelSearchKind /v <your_app.exe> /t REG_DWORD /d 1`
+3. Restart your application, and you should be able to see that your application picks up the latest runtime. 
+
+To remove the registry key added, run the following:  
+`REG DELETE HKCU\Software\Policies\Microsoft\Edge\WebView2\ChannelSearchKind`
+
+For more information, read the documentation on [Test upcoming APIs and features](https://learn.microsoft.com/en-us/microsoft-edge/webview2/how-to/set-preview-channe)