Add basic instructions for using xAI Grok.

[pieterza]

I wanted to see if xAI's grok is supported, but could not find any existing docs/pull-requests/issues/discussions or instructions.

https://x.ai/blog/api

API
To learn more about our API, check out our documentation at [docs.x.ai](https://docs.x.ai/). Our REST API is fully compatible with the ones offered by OpenAI and Anthropic. This simplifies migration. For example, if you’re currently using the OpenAI Python SDK, you can start using Grok by simply changing the base_url to https://api.x.ai/v1 and using your xAI API key that you created on [console.x.ai](https://console.x.ai/).
The xAI API is 

As such I've figured out how to make it work and this PR adds instructions accordingly:

export OPENAI_API_KEY='xai-xxxx'
export DEFAULT_MODEL="grok-beta" #Or whatever model is listed at the time, and preferred.
export API_BASE_URL=https://api.x.ai/v1