When an env variable is used for the API key, include it in the SDK docs

[jcbwndsr]

Which Fern component?

SDK Generator

How important is this?

P0 - Critical (Blocking work)

What's the feature?

Feature Include env variable usage in the API key docs
Why? So that users of the generated SDK know that they can use the docs

Description
Now, when an environment variable is allowed for setting the client, API key, it is undocumented in the generated README.

generators.yml

...
auth-schemes:
  ApiKeyAuthScheme:
    header: X-API-Key
    type: string
    name: apiKey
    env: PLANTS_API_KEY  
...

resulting README.md:

from axiomatic import Axiomatic, UserRequirement

client = Axiomatic(
    api_key="YOUR_API_KEY",
)

Suggested README addition to "usage" section

### Environment Variable
You may also set the API key via an environment variable:

```python
from axiomatic import Axiomatic, UserRequirement
import os

os.environ["PLANTS_API_KEY"] = "YOUR_API_KEY"

client = Axiomatic()

Are you interested in contributing this feature?

Can do!