Add notes on testing the link

Author: code-asher

CONTRIBUTING.md

@@ -44,3 +44,15 @@ Coder Remote periodically reads the `network-info-dir + "/" + matchingSSHPID` fi
    ```
 
 3. Press `F5` or navigate to the "Run and Debug" tab of VS Code and click "Run Extension".
+
+There are a few ways you can test the "Open in VS Code" flow:
+
+- Use the "VS Code Desktop" button from a Coder dashboard.
+- Manually open the link with `Developer: Open URL` from inside VS Code.
+- Use `code --open-url <link>` on the command line.
+
+The link format is `vscode://coder.coder-remote/open?${query}`.  For example:
+
+```
+code --open-url 'vscode://coder.coder-remote/open?url=dev.coder.com&owner=my-username&workspace=my-ws&agent=my-agent'
+```