docs: add debugging command

Author: averikitsch

README.md

@@ -79,6 +79,11 @@ Find additional extensions to support your entire software development lifecycle
 
 ## Troubleshooting
 
+Use `gemini --debug` to enable debugging.
+
+Common issues:
+
+* "failed to find default credentials: google: could not find default credentials.": Ensure [Application Default Credentials](https://cloud.google.com/docs/authentication/gcloud) are available in your environment. See [Set up Application Default Credentials](https://cloud.google.com/docs/authentication/external/set-up-adc) for more information.
 * "✖ Error during discovery for server: MCP error -32000: Connection closed": The database connection has not been established. Ensure your configuration is set via environment variables.
-* "✖ MCP ERROR: Error: spawn /Users/<USER>/.gemini/extensions/spanner/toolbox ENOENT": The Toolbox binary did not download correctly. Ensure you are using Gemini CLI v0.6.0+.
+* "✖ MCP ERROR: Error: spawn /Users/USER/.gemini/extensions/spanner/toolbox ENOENT": The Toolbox binary did not download correctly. Ensure you are using Gemini CLI v0.6.0+.
 * "cannot execute binary file": The Toolbox binary did not download correctly. Ensure the correct binary for your OS/Architecture has been downloaded. See [Installing the server](https://googleapis.github.io/genai-toolbox/getting-started/introduction/#installing-the-server) for more information.