docs: how to log messages between code and ls

Author: wkillerud

docs/src/language-server/configure-a-client.md

@@ -12,5 +12,15 @@ The language server requests [settings](../user-guide/settings.md) via the [`wor
 
 This list of [language client implementations][languageclients] may be a helpful starting point. You may also want to look at [existing clients](./existing-clients.md).
 
+### Log messages sent by VS Code to the server
+
+If you're having trouble it might be helpful to compare your client with VS Code's. To log the messages sent between VS Code and the language server, add this to [your `settings.json`](https://code.visualstudio.com/docs/getstarted/settings#_settingsjson) (thank you to [Stefan Schlichthärle](https://www.sscit.de/2021/04/15/trace-lsp-in-vscode.html)).
+
+```json
+"some-sass.trace.server": "verbose"
+```
+
+Now you can open a Sass file, then open the Output panel (View menu -> Output) to see the messages.
+
 [lsp]: https://microsoft.github.io/language-server-protocol/
 [languageclients]: https://microsoft.github.io/language-server-protocol/implementors/tools/