add cli intro

docs/cli/index.md

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+# Instructor CLI
+
+Welcome to the Instructor Command-Line Interface (CLI), a tool designed to ease your experience with the OpenAI API. Whether it's tracking your API usage or fine-tuning your models, Instructor CLI is your go-to utility.
+
+---
+
+## Quick Start
+
+First things first: make sure your OpenAI API key is set as an environment variable. The CLI will use this for authenticating your requests to OpenAI's services.
+
+You can set the API key in your terminal as follows:
+
+```bash
+export OPENAI_API_KEY="your-api-key-here"
+```
+
+## Installation & Setup
+
+```bash
+pip install instructor
+```
+
+---
+
+## Features
+
+- **API Usage Monitoring**: Keep tabs on your API usage right from the terminal. Track token counts, total requests, and even calculate the costs. To learn more, consult the [Usage Guide](usage.md).
+  
+- **Model Fine-Tuning**: Optimize your models to meet your specific requirements using our fine-tuning app. For more details, check out the [Fine-Tuning Guide](finetune.md).
+
+---
+
+## Support & Contribution
+
+Need help or want to contribute? Visit our [GitHub Repository](https://github.com/jxnl/instructor)