docs: add warning about Gemini OAuth policy risk

Author: jenslys

README.md

@@ -3,9 +3,19 @@
 ![License](https://img.shields.io/npm/l/opencode-gemini-auth)
 ![Version](https://img.shields.io/npm/v/opencode-gemini-auth)
 
+> [!WARNING]
+> Google has stated that using Gemini CLI OAuth with third-party software is a
+> policy-violating use case and may trigger abuse detection or account
+> restrictions. It is unclear how aggressively this is enforced for projects
+> like this one, but you should assume there is real risk and use this plugin at
+> your own discretion. If you want the lowest-risk option, use Opencode with
+> your own Gemini API key instead.
+>
+> See: https://github.com/google-gemini/gemini-cli/discussions/22970
+
 **Authenticate the Opencode CLI with your Google account.** This plugin enables
 you to use your existing Gemini plan and quotas (including the free tier)
-directly within Opencode, bypassing separate API billing.
+directly within Opencode.
 
 ## Prerequisites