Update README.md to mention this is an Unofficial package

[logankilpatrick]

Thanks!

[codecov]

Codecov Report

Merging #245 (8db3a6a) into master (d94c5e7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #245   +/-   ##
=======================================
  Coverage   91.03%   91.03%           
=======================================
  Files          22       22           
  Lines         591      591           
=======================================
  Hits          538      538           
  Misses         39       39           
  Partials       14       14           

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[sashabaranov]

@logankilpatrick thank you for the PR! Do you mind putting this not in the title, but in the first line of the description, e.g.

This library provides unofficial Go clients for [OpenAI API](https://platform.openai.com/). We support: 
…

[sashabaranov]

Closed in #319