Add xai plugin to README agents list (#994)

Author: gianpaj

README.md

@@ -76,6 +76,7 @@ Currently, only the following plugins are supported:
 | [@livekit/agents-plugin-livekit](https://www.npmjs.com/package/@livekit/agents-plugin-livekit)       | EOU           |
 | [@livekit/agents-plugin-anam](https://www.npmjs.com/package/@livekit/agents-plugin-anam)             | Avatar        |
 | [@livekit/agents-plugin-bey](https://www.npmjs.com/package/@livekit/agents-plugin-bey)               | Avatar        |
+| [@livekit/agents-plugin-xai](https://www.npmjs.com/package/@livekit/agents-plugin-xai)               | LLM, TTS      |
 
 ## Docs and guides