howto add key

[bjornjorgensen]

Hi, I did have some problems with setting up the API key in a jupyter notebook. Like in #315

This in a code cell does work for chatgpt.

import os
os.environ["OPENAI_API_KEY"]='sk-'

[welcome]

Thanks for submitting your first pull request! You are awesome! 🤗

If you haven't done so already, check out Jupyter's Code of Conduct. Also, please make sure you followed the pull request template, as this will help us review your contribution more quickly.

You can meet the other Jovyans by joining our Discourse forum. There is also a intro thread there where you can stop by and say Hi! 👋

Welcome to the Jupyter community! 🎉

[JasonWeill]

This is a partial fix for #315.

[JasonWeill]

@bjornjorgensen Thank you very much for opening this pull request! Because configuration is required for all of our model providers, it might be better to modify the Requirements section of README.md to mention the requirement to configure your models, as suggested in #315:

## Requirements
- Python 3.8 - 3.11
- JupyterLab 4

You will need API keys or other credentials for each model provider you use
with Jupyter AI. For information about credentials and how to provide them, see
[the model providers documentation](https://jupyter-ai.readthedocs.io/en/latest/users/index.html#model-providers).

[bjornjorgensen]

what if we change the text to

## Setting Up Model Providers in a Notebook

To use any AI model provider within this notebook, you'll need the appropriate credentials, such as API keys.

Obtain the necessary credentials (e.g., API keys) from your model provider's platform.

In a new cell, set the credentials as follows:

import os

# NOTE: Replace 'YOUR_CREDENTIAL_HERE' with your actual credential.
# And replace 'YOUR_ENVIRONMENT_VARIABLE' with the appropriate name.
os.environ["YOUR_ENVIRONMENT_VARIABLE"] = 'YOUR_CREDENTIAL_HERE'

If you're using ChatGPT from OpenAI, it would look like:
import os
os.environ["OPENAI_API_KEY"] = 'YOUR_API_KEY_HERE'

Adjust the environment variable name and the placeholder for your specific model provider. 
For more specific instructions for each model provider, refer to the model providers documentation.

[JasonWeill]

A few minor suggestions, but otherwise looks good. Thank you very much for your contributions!

[welcome]

Congrats on your first merged pull request in this project! 🎉

Thank you for contributing, we are very proud of you! ❤️