Adding some troubleshooting text for login issues (#1451)

README.md

@@ -80,6 +80,11 @@ gemini
 - For more comprehensive documentation, see the [full documentation](./docs/index.md).
 - Take a look at some [popular tasks](#popular-tasks) for more inspiration.
 
+### Troubleshooting
+
+Head over to the [troubleshooting](docs/troubleshooting.md) guide if you're
+having issues.
+
 ## Popular tasks
 
 ### Explore a new codebase

docs/troubleshooting.md

@@ -2,6 +2,19 @@
 
 This guide provides solutions to common issues and debugging tips.
 
+## Authentication
+
+- **Error: `Failed to login. Message: Request contains an invalid argument`**
+
+  - Users with Google Workspace accounts, or users with Google Cloud accounts
+    associated with their Gmail accounts may not be able to activate the free
+    tier of the Google Code Assist plan.
+  - For Google Cloud accounts, you can work around this by setting
+    `GOOGLE_CLOUD_PROJECT` to your project ID.
+  - You can also grab an API key from [AI
+    Studio](http://aistudio.google.com/app/apikey), which also includes a
+    separate free tier.
+
 ## Frequently asked questions (FAQs)
 
 - **Q: How do I update Gemini CLI to the latest version?**