Update README to include Gemini instructions

[francis]

Google Gemini Pro has an OpenAI-compatible API that works with this gem. I added a section in the README showing how to configure your gem to use Gemini.

All Submissions:

• [X ] Have you followed the guidelines in our Contributing document?
• [ X] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ X] Have you added an explanation of what your changes do and why you'd like us to include them?

[mikailbayram]

Was definitely looking for this bit of documentation, thanks @francis

[alexrudall]

Released in v8.2.0 - thanks @francis