Update README.md (#319)

Added in `unofficial` to the README to make it clear it's not official.

Author: TomHennessy

README.md

@@ -3,7 +3,7 @@
 [![Go Report Card](https://goreportcard.com/badge/github.com/sashabaranov/go-openai)](https://goreportcard.com/report/github.com/sashabaranov/go-openai)
 [![codecov](https://codecov.io/gh/sashabaranov/go-openai/branch/master/graph/badge.svg?token=bCbIfHLIsW)](https://codecov.io/gh/sashabaranov/go-openai)
 
-This library provides Go clients for [OpenAI API](https://platform.openai.com/). We support:
+This library provides unofficial Go clients for [OpenAI API](https://platform.openai.com/). We support: 
 
 * ChatGPT
 * GPT-3, GPT-4